Rethinking configuration.

There are 3 ways of passing `reek` a configuration file:

1. Using the cli "-c" switch
2. Having a file ending with .reek either in your current working directory or in a parent directory (more on that later)
3. Having a file ending with .reek in your HOME directory

The order in which `reek` tries to find such a configuration file is exactly like above: First `reek` checks if we have given it a configuration file explicitly via CLI. Then it checks the current working directory for a file and if it can't find one, it traverses up the directories until it hits the root directory. And lastly, it checks your HOME directory.

As soon as `reek` detects a configuration file it stops searching immediately, meaning that from `reek`'s point of view there exists one configuration file and one configuration only regardless of how many ".reek" files you might have on your filesystem.
Merge pull request #356 from troessner/rethinking-configuration

Rethinking configuration.
Bump version to 1.6.0.
Support Ruby 2.2

- Add 2.2 to the Travis CI configuration
- Fix warnings