Perform gem install bundler to address `LoadError: cannot load such

file -- bundler/dep_proxy`

```ruby
bundle exec rake
bundler: failed to load command: rake (/home/travis/build/mikel/mail/vendor/bundle/ruby/2.5.0/bin/rake)
LoadError: cannot load such file -- bundler/dep_proxy
```

.travis.yml

@@ -4,7 +4,7 @@ cache: bundler
 
 bundler_args: --without local_development
 before_install:
-  - gem update bundler
+  - gem install bundler
 
 script: bundle exec rake