fix failing travis ci builds

travis ci builds are failing with libv8 compilation errors.
It might be because bundler 2.2 was released yesterday and we've
been installing bundler on travis with "--pre".

Something seems to be up with bundler 2.2 being "smart" about fetching
pre-built gems. Maybe.

rubyjs/libv8#310 (comment)

.travis.yml

@@ -9,7 +9,7 @@ sudo: required
 bundler_args: --without production
 before_install:
   - gem update --system
-  - gem install bundler --pre
+  - gem install -v 2.1.4 bundler
   - nvm install node
   - npm install --global yarn
 install: