Should fix the travis errors

This should fix all travis errors
Details:
* `.travis.yml` travis has a bug with `rbx` "_When using Rubinius, there's currently an issue with selecting the correct version in RVM in our build environment, but only when specifying rbx as your version. As a workaround, specify rbx-2 instead._"

* `Gemfile` checking if the rails  environment is either master or more than 4.1(regex) to require minitest 5.1 otherwise keep the old minitest version

* `slim_test.rb`: there was a problem with rails 3.1.x and jruby-mode19 when streaming


* `gitignore`: I'am using rubymine just added the config folder to gitignore

I don't think that's an problem with slim, I rendered a erb layout and a erb view with streaming and  jruby and still got the same error
```
FiberError (dead fiber called):
  org/jruby/ext/fiber/ThreadFiber.java:58:in `resume'
```

I also tried to go back to a previous version of jruby where the test was working(1.7.9) and it worked. So I think it's a problem with rails and jruby that keep happening.
For example jruby 1.7.9 was not working with rails [3.1.15](https://travis-ci.org/slim-template/slim/jobs/23289235) but was with [3.1.17](https://travis-ci.org/slim-template/slim/jobs/23374213) then jruby 1.7.11 broke again with [3.1.17](https://travis-ci.org/slim-template/slim/jobs/27009764)

We can also put jruby-19mode in the Allowed Failures list.
What do you think?