Modernazing and cleanup

Would you be open to pull requests to "modernize" the gem a bit ?

By modernizing, I mean:

* Use `gemspec` in Gemfile to reduce duplication
* Clean up the the _gemspec_, remove Jeweler dependency and simplify it (the bundler way)
* Add Travis-CI, CodeClimate, Gemnasium and CoverAll
* Clean up the specs to make it more "rspec-way"

I'd also clean up the Wiki to refresh it a bit concerning Rails and Ruby versions.

I'd really love to see simple-navigation keep up with the community. It's an awesome project and I can give some time to help :)