.travis.yml: pass tests with 2.2.2
.travis.yml: pass tests with 2.2.2

I thought that specifying 2.2 would make Travis use the latest version in
the 2.2.X branch, but it looks like it uses 2.2.0, which is not
supported anymore, as the version of rack used requires >= 2.2.2.

davidor/formulita-backend,davidor/formulita-backend