.travis.yml: remove ruby-head untill fix travis bug(?)
.travis.yml: remove ruby-head untill fix travis bug(?)

cf. https://github.com/travis-ci/travis-ci/issues/5798

krororo/review,takahashim/review,krororo/review,krororo/review,takahashim/review,orangain/review,orangain/review,kmuto/review,kmuto/review,kmuto/review