Build seems to be broken

[vguerlesquin]

I've noticed that Ruby version in dockerfile was higher than expected, generating a fail when trying to build the image.
I've fixed it and tryied to PR my changes.

1. In Travis CI, build seems to need a lower version of Ruby than the one originally specified. I've unified all of this (Ruby version in Dockerfile, Gemfile and Gemfile.lock, same for Bundler version)
2. "bundle exec rake" command in Travis job fail for a reason I don't now or master. Then, the build is broken. Can anyone fix it ?

I think the fix should allow PR 66 (#66) to build correctly.