Remove temporary workaround for expected HHVM test failures on memcache

HHVM 3.4 (which resolves the incompatible memcache behaviour discussed
in #52) is now available on Travis so this workaround can come out.

This reverts commit b9f7c082cde450755a3238c17e89633120d8a67a.
Merge pull request #64 from kohana/3.3/feat/reinstate-hhvm-tests

Remove temporary workaround for expected HHVM test failures on memcache