Update almost all gems (second try)

Almost everything went ok. But now we're stuck with sinatra 1.3.6 because
sinatra-authorization don't seems to work as is with 1.4.x.

I'll see if I can work around this or if we need to fix/get rid of
this dependency.
Update to sinatra 1.4.2
Merge pull request #239 from Planbox/update-gems

Update almost all gems (second try)