Add new virtual variant 'neutral'.
Ignore --with-libdir compile option in testNeutralVirtualVarint method.

Whether the compile option is used or not depends on client environments.
Remove the HHVM build from .travis.yml

The HHVM build currently makes our builds failed because there is
no phpize command in HHVM build's environment on Travis CI.
Futheremore, almost all PHP extensions only support PHP implemented by
Zend.
Merge pull request #245 from shinnya/feature/neutral_virtual_variant

Add a new virtual variant 'neutral'.