Revert rubygems 2.0.0 requirement.

Seems it's not necessary given the facilities used in the gem, and imposes a burden on users to update.

In our case, we currently can't run our CI on tddium.

Was introduced here: 9e2a9fccef2d1a9480efebf81343a44826d10469
Add @Empact to contributors
v0.14.2