Adds XML linter

Fixes webimpress#80

.travis.yml

@@ -21,6 +21,11 @@ matrix:
         - LEGACY_DEPS="phpunit/phpunit"
         - CS_CHECK=true
         - TEST_COVERAGE=true
+        - XMLLINT=true
+      addons:
+        apt:
+          packages:
+            - libxml2-utils
     - php: 7.1
       env:
         - DEPS=latest
@@ -67,6 +72,7 @@ install:
 script:
   - if [[ $TEST_COVERAGE == 'true' ]]; then composer test-coverage ; else composer test ; fi
   - if [[ $CS_CHECK == 'true' ]]; then composer cs-check ; fi
+  - if [[ $XMLLINT == 'true' ]]; then test/xmllint.sh ; fi
 
 after_script:
   - if [[ $TEST_COVERAGE == 'true' ]]; then travis_retry php vendor/bin/php-coveralls -v ; fi

ruleset.xml

@@ -1,4 +1,4 @@
 <?xml version="1.0"?>
-<ruleset name="Webimpress Coding Standard">
+<ruleset xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" name="WebimpressCodingStandard" xsi:noNamespaceSchemaLocation="vendor/squizlabs/php_codesniffer/phpcs.xsd">
     <rule ref="vendor/webimpress/coding-standard/src/WebimpressCodingStandard/ruleset.xml"/>
 </ruleset>

src/WebimpressCodingStandard/ruleset.xml

@@ -1,7 +1,5 @@
 <?xml version="1.0"?>
-<ruleset name="Webimpress Coding Standard"
-         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
-         xsi:noNamespaceSchemaLocation="../../vendor/squizlabs/php_codesniffer/phpcs.xsd">
+<ruleset xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" name="WebimpressCodingStandard" xsi:noNamespaceSchemaLocation="../../vendor/squizlabs/php_codesniffer/phpcs.xsd">
     <description>Webimpress Coding Standard</description>
 
     <!-- display progress -->

test/xmllint.sh

@@ -0,0 +1,18 @@
+#!/bin/bash
+
+RULESET="src/WebimpressCodingStandard/ruleset.xml"
+LOCAL="ruleset.xml"
+
+set -e
+
+wget -nv -N -P tmp/ "https://github.com/squizlabs/PHP_CodeSniffer/raw/master/phpcs.xsd"
+wget -nv -N -P tmp/ "https://www.w3.org/2012/04/XMLSchema.xsd"
+
+xmllint --noout --schema tmp/XMLSchema.xsd tmp/phpcs.xsd
+xmllint --noout --schema tmp/phpcs.xsd "$RULESET"
+diff -B "$RULESET" <(XMLLINT_INDENT="    " xmllint --format "$RULESET")
+
+xmllint --noout --schema tmp/phpcs.xsd "$LOCAL"
+diff -B "$LOCAL" <(XMLLINT_INDENT="    " xmllint --format "$LOCAL")
+
+rm -Rf tmp/