update bundler on travis
update bundler on travis

Warning: the running version of Bundler (1.16.0) is older than the version that created the lockfile (1.16.1). We suggest you upgrade to the latest version of Bundler by running `gem install bundler`.
https://travis-ci.org/paulz/PerspectiveTransform/jobs/347584158

paulz/PerspectiveTransform,paulz/PerspectiveTransform,paulz/PerspectiveTransform