Update Travis' System Gems
Update Travis' System Gems

Update system gems before install to fix issue from CI:

```
bundler: failed to load command: rake (/home/travis/build/tech-angels/vandamme/vendor/bundle/ruby/2.6.0/bin/rake)
LoadError: cannot load such file -- bundler/dep_proxy
```

https://github.com/travis-ci/travis-ci/issues/9333#issuecomment-373042916

thoughtbot/json_matchers,seanpdoyle/json-matchers