follow maintaining versions of Ruby and Rails

Rails 3.1 is outdated and no longer supported and Rails 4.2 will release soon.
Also Ruby 1.9.3 will end on February 23, 2015 and Ruby 2.2 will release in few months.

I suggest Doorkeeper focusing testing and supporting maintaining versions of Ruby and Rails, that will make our Travis-CI faster.
This PR upgrade default development environment to Rails 4.1, and Travis-CI no longer test Rails 3.1(although Doorkeeper works well with it).
That is right. If you do:
```bash
$ rails=4.1 rake
```
or
```bash
$ export rails=4.1
$ rake
```
that assignment won't happen. It's only there as a convenience for running `rake` without specifying options.

Travis always sets (or should set!) all the configuration variables.