Fix gem dependencies

Thor ~> 0.1.0 is not available from rubygems.

Bump the dependency to the most recent minor.

Kramdown would install 1.0.x, but 1.3 should be necessary.

Relaxed minitest dependency to 5.0.0 and higher.
Merge pull request #78 from skade/correct-thor-dependency

Bump thor dependency to ~> 0.19.0