[BUGFIX] Use a different EmptyStatement Sniff

PHP_CodeSniffer 2.1 has removed the Squiz EmptyStatement Sniff,
causing the build in Travis CI to fail.

This change switches our coding standard to the Generic EmptyStatement Sniff.
Merge pull request #147 from jjriv/bugfix/codesniffer

[BUGFIX] Use a different EmptyStatement Sniff