Update bundler before in Travis CI builds

Builds are broken because the `bundler` version that Travis CI
automatically uses at this time (`1.7.6`) is coming across an
error that was fixed in a later version/release.

Having an outdated version of `bundler` in the Travis CI builds is an
ongoing issue and this is currently the easiest work-around.

See:

* rubygems/bundler#3558
* travis-ci/travis-ci#3531

[fixes #846]

.travis.yml

@@ -7,6 +7,7 @@ rvm:
   - rbx-2
 before_install:
   - gem update --system
+  - gem update bundler
 install: "bundle install"
 script: "bundle exec rake spec:unit spec:acceptance features"
 jdk: