incompatible bundler versions

[kdiogenes]

Hi,

When I try to bundle this repo I get the following error:

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 (~> 1.16)

  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 (~> 1.16)' in any of the relevant sources:
  the local ruby installation

I had to change the gemspec to spec.add_development_dependency "bundler", "~> 2.1.4" to make it work. Removing bundler from gemspec also solves the problem. Is it really necessary to be specified in the gemspec?