bundler restricted to version <2.0

[sherwinski]

imgix-rails.gemspec is pinning the version of bundler between 1.9 and 2.0, not inclusive. This will require anyone running version 2.0 (released at the beginning of the year) to downgrade in order to successfully build. The gemspec file should be modified to allow for newer versions of bundler.

spec.add_development_dependency "bundler", "~> 1.9"

Steps to reproduce:

$ git clone https://github.com/imgix/imgix-rails.git
...
$ cd imgix-rails/
$ bundler --version
Bundler version 2.0.1
$ bin/setup
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 (~> 1.9)

  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 (~> 1.9)' in any of the relevant sources:
  the local ruby installation