Composer: require phpseclib 2.0.47

phpseclib 2.0.47 does not introduce any new features that mcrypt_compat
makes use of, however, in some cases, it might be benificial to have
new releases to prevent phpseclib versions with CVEs issued against
them from ever being used as dependencies as discussed in
#37

composer.json

@@ -22,7 +22,7 @@
     },
     "require": {
         "php": ">=5.3.3",
-        "phpseclib/phpseclib": ">=2.0.36 <3.0.0"
+        "phpseclib/phpseclib": ">=2.0.47 <3.0.0"
     },
     "require-dev": {
         "phpunit/phpunit": "^4.8.35|^5.7|^6.0|^9.4"