Specify Bundler version for CI
Specify Bundler version for CI

The jruby build is failing to install a dependency properly:
https://travis-ci.org/craiglittle/hello_sign/jobs/16282109

Apparently, locking to this version of Bundler solves the issue.

See: https://github.com/travis-ci/travis-ci/issues/1798

craiglittle/hello_sign,craiglittle/hello_sign