Fix Travis CI build status image.
Fix Travis CI build status image.

The image is not displayed properly because Travis SSL uses the
Heroku wildcard SSL certificate and is thus not trusted because
of the URL mismatch.

sideci-sample/sideci-sample-guard,yujinakayama/guard,100star/guard,pjump/guard,ginkel/guard,sdgdsffdsfff/guard,garettarrowood/guard,guard/guard,baberthal/guard