Bundler could not find compatible versions for gem "bundler"

[etagwerker]

Hi @bleonard,

Thanks for maintaining this project! It has been very useful so far.

I was interested in making my first contribution to your project, but I noticed that it is requiring an old version of bundler:

$ bundle
Fetching gem metadata from https://rubygems.org/.
Fetching gem metadata from https://rubygems.org/.
Resolving dependencies...
Bundler could not find compatible versions for gem "bundler":
  In Gemfile:
    bundler (~> 1.6)

  Current Bundler version:
    bundler (2.1.4)
This Gemfile requires a different version of Bundler.
Perhaps you need to update Bundler by running `gem install bundler`?

Could not find gem 'bundler (~> 1.6)' in any of the relevant sources:

Is there a good reason to require bundler v1.6.x instead of any bundler?

Things work fine if I relax the dependency.

Please let me know.

Thanks!