Allow testing with bundler 2

[deivid-rodriguez]

Otherwise I get the error:

$ bundle exec thor spec
Bundler could not find compatible versions for gem "bundler":
  In Gemfile:
    bundler (~> 1.0)

  Current Bundler version:
    bundler (2.0.1)
This Gemfile requires a different version of Bundler.
Perhaps you need to update Bundler by running `gem install bundler`?

Could not find gem 'bundler (~> 1.0)' in any of the relevant sources:
  the local ruby installation

[deivid-rodriguez]

CI needs #649 to get to green.

[deivid-rodriguez]

Closing in favor of #649. I added this change to that PR to make things simpler for myself in order to pick up both patches locally.