travis: Add rubex directory to PKG_CONFIG_PATH search directory.
travis: Add rubex directory to PKG_CONFIG_PATH search directory.

As apparently libonig-dev doesn't contain a .pc file. Should fix
issue #263.

ch1bo/lime-backend,bj7/lime-backend,limetext/backend,ch1bo/lime-backend,ch1bo/lime-backend,codex8/lime-backend,limetext/lime-backend,bj7/lime-backend,limetext/lime-backend,codex8/lime-backend,codex8/lime-backend,bj7/lime-backend,codex8/lime-backend,limetext/lime-backend,limetext/lime-backend,ch1bo/lime-backend,bj7/lime-backend