[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
[TASK] Add sudo: false to .travis.yml

This allows Travis CI to use the new container-based infrastructure,
which should speed up the builds.

Closes #148