Fix Travis build for Rubinius

Installing gems with RubyGems <= 2.4.0 is not thread-safe, and because of that Rubinius was failing when installing gems. The latest RubyGems, 2.4.1, allegedly fixes that problem, so I added

```sh
$ gem update --system 2.4.1
```

to the Travis build. But that didn't work, the error was still there. So I decided to just not install gems in parallel, and cache the installed gems, so that step should still be fast enough.