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.

farhaanbukhsh/lime,crr0004/lime,farhaanbukhsh/lime,crr0004/lime,farhaanbukhsh/lime,crr0004/lime,crr0004/lime,farhaanbukhsh/lime,crr0004/lime,farhaanbukhsh/lime