Composer: set minimum PHPCS 3.x version to ^3.1.0

When `installed_paths` is set via a custom, the PHPCS native autoloader cannot find non-sniff files provided by an external standard, including the PHPCSUtils files.

As PHPCS 3.0.2 is rarely used anymore anyway, let's just not support it.

Ref: squizlabs/PHP_CodeSniffer#1591

composer.json

@@ -21,7 +21,7 @@
     },
     "require" : {
         "php" : ">=5.4",
-        "squizlabs/php_codesniffer" : "^2.6.0 || ^3.0.2",
+        "squizlabs/php_codesniffer" : "^2.6.0 || ^3.1.0",
         "dealerdirect/phpcodesniffer-composer-installer" : "^0.5"
     },
     "require-dev" : {