Update bundler before 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Having an outdated version of `bundler` in the Travis CI builds is an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:

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

(commit msg taken from: 3c0547fa50)

.travis.yml

@@ -1,4 +1,8 @@
+# Use container-based infrastructure
+sudo: false
 language: ruby
+# Travis bug, see https://github.com/bundler/bundler/pull/3559
+before_install: gem update bundler
 cache: bundler
 rvm:
   - ruby-head