Build/PHPCS: Rename PHPCS configuration file

This rename has two functions:
1. Using a `.dist` extension allows for individual developers to overload the `phpcs.xml.dist` with a local `phpcs.xml` file. This file can include the `dist` file as a base ruleset using `<rule ref="./phpcs.xml.dist"/>`.
2. Since [PHPCS 3.1.0](https://github.com/squizlabs/PHP_CodeSniffer/releases/tag/3.1.0) allows for config files to start with a `.`.
    We can use this feature to have a separate ruleset for the Free and the Premium plugin.
    The loading order of the files is `.phpcs.xml`, `phpcs.xml`, `.phpcs.xml.dist`, `phpcs.xml.dist` (as of [PHPCS 3.1.1](https://github.com/squizlabs/PHP_CodeSniffer/releases/tag/3.1.1)).
    By naming the config file for WPSEO Free `phpcs.xml.dist`, we can automatically overrule that config in the Premium plugin by having the Premium PHPCS config be called `.phpcs.xml.dist`.

To facilitate (1), the typical file names for the overloaded PHPCS config files have been added to the `.gitignore` and `.gitattributes` files.

.gitattributes

@@ -22,7 +22,9 @@ webpack export-ignore
 .travis.yml export-ignore
 Gruntfile.js export-ignore
 package.json export-ignore
+.phpcs.xml export-ignore
 phpcs.xml export-ignore
+.phpcs.xml.dist export-ignore
 phpcs.xml.dist export-ignore
 phpdoc.xml export-ignore
 phpdoc.dist.xml export-ignore

.gitignore

@@ -21,6 +21,8 @@ nbproject/
 node_modules/
 bower_components/
 premium/node_modules/
+phpcs.xml
+.phpcs.xml
 
 ############
 ## OSes

grunt/config/phpcs.js

@@ -6,7 +6,7 @@ module.exports = {
 	plugin: {
 		options: {
 			bin: "vendor/bin/phpcs",
-			standard: "phpcs.xml",
+			standard: "phpcs.xml.dist",
 			extensions: "php",
 		},
 		dir: [