Update phpunit/phpunit requirement from 6.5.8 to 6.5.13 (#4)

Updates the requirements on [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) to permit the latest version.
<details>
<summary>Changelog</summary>

*Sourced from [phpunit/phpunit's changelog](https://github.com/sebastianbergmann/phpunit/blob/6.5.13/ChangeLog-6.5.md).*

> ## [6.5.13] - 2018-09-08
> 
> * Fixed [#3181](https://github-redirect.dependabot.com/sebastianbergmann/phpunit/issues/3181): `--filter` should be case-insensitive
> * Fixed [#3234](https://github-redirect.dependabot.com/sebastianbergmann/phpunit/issues/3234): `assertArraySubset()` with `$strict=true` does not display differences properly
> * Fixed [#3254](https://github-redirect.dependabot.com/sebastianbergmann/phpunit/issues/3254): TextUI test runner cannot run a `Test` instance that is not a `TestSuite`
> 
> ## [6.5.12] - 2018-08-22
> 
> * Fixed [#3248](https://github-redirect.dependabot.com/sebastianbergmann/phpunit/issues/3248) and [#3233](https://github-redirect.dependabot.com/sebastianbergmann/phpunit/issues/3233): `phpunit.xsd` dictates element order where it should not
> * Fixed [#3251](https://github-redirect.dependabot.com/sebastianbergmann/phpunit/issues/3251): TeamCity result logger is missing test duration information
> 
> ## [6.5.11] - 2018-08-07
> 
> * Fixed [#3219](https://github-redirect.dependabot.com/sebastianbergmann/phpunit/issues/3219): `getMockFromWsdl()` generates invalid PHP code when WSDL filename contains special characters
> 
> ## [6.5.10] - 2018-08-03
> 
> ### Fixed
> 
> * Fixed [#3209](https://github-redirect.dependabot.com/sebastianbergmann/phpunit/issues/3209): `Test::run()` and `TestCase::run()` interface contradiction
> * Fixed [#3218](https://github-redirect.dependabot.com/sebastianbergmann/phpunit/issues/3218): `prefix` attribute for `directory` node missing from `phpunit.xml` XSD
> * Fixed [#3225](https://github-redirect.dependabot.com/sebastianbergmann/phpunit/issues/3225): `coverage-php` missing from `phpunit.xsd`
> 
> ## [6.5.9] - 2018-07-03
> 
> ### Fixed
> 
> * Fixed [#3142](https://github-redirect.dependabot.com/sebastianbergmann/phpunit/issues/3142): Method-level annotations (`[**backupGlobals**](https://github.com/backupGlobals)`, `[**backupStaticAttributes**](https://github.com/backupStaticAttributes)`, `[**errorHandler**](https://github.com/errorHandler)`, `[**preserveGlobalState**](https://github.com/preserveGlobalState)`) do not override class-level annotations
> 
> ## [6.5.8] - 2018-04-10
> 
> ### Fixed
> 
> * Fixed [#2830](https://github-redirect.dependabot.com/sebastianbergmann/phpunit/issues/2830): `[**runClassInSeparateProcess**](https://github.com/runClassInSeparateProcess)` does not work for tests that use `[**dataProvider**](https://github.com/dataProvider)`
> 
> ## [6.5.7] - 2018-02-26
> 
> ### Fixed
> 
> * Fixed [#2974](https://github-redirect.dependabot.com/sebastianbergmann/phpunit/issues/2974): JUnit XML logfile contains invalid characters when test output contains binary data
> 
> ## [6.5.6] - 2018-02-01
> 
> ### Fixed
> 
> * Fixed [#2236](https://github-redirect.dependabot.com/sebastianbergmann/phpunit/issues/2236): Exceptions in `tearDown()` do not affect `getStatus()`
> * Fixed [#2950](https://github-redirect.dependabot.com/sebastianbergmann/phpunit/issues/2950): Class extending `PHPUnit\Framework\TestSuite` does not extend `PHPUnit\FrameworkTestCase`
> * Fixed [#2972](https://github-redirect.dependabot.com/sebastianbergmann/phpunit/issues/2972): PHPUnit crashes when test suite contains both `.phpt` files and unconventionally named tests
> 
> ## [6.5.5] - 2017-12-17
></table> ... (truncated)
</details>
<details>
<summary>Commits</summary>

- See full diff in [compare view](https://github.com/sebastianbergmann/phpunit/commits/6.5.13)
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composer.json

@@ -8,6 +8,6 @@
         "squizlabs/php_codesniffer": "3.3.2"
     },
     "require-dev": {
-        "phpunit/phpunit": "6.5.8"
+        "phpunit/phpunit": "6.5.13"
     }
 }