Can't install with development gem set due to pinned bundler version

[MatzFan]

$ git clone git@github.com:bbtfr/evil-proxy.git
$ cd evil-proxy/
$ bundle install
Resolving dependencies...
Could not find compatible versions

Because the current Bundler version (2.7.0.dev) does not satisfy bundler ~> 1.6
  and Gemfile depends on bundler ~> 1.6,
  version solving has failed.

Your bundle requires a different version of Bundler than the one you're running.
Install the necessary version with `gem install bundler:1.17.3` and rerun bundler using `bundle
_1.17.3_ install`

This line in the gemspec is the problem. As Bundler has been part of Ruby since 2.6.0 that line is unnecessary. If I remove it I can run bundler, install the development gems and run the tests.