Remove non-required Rails version requirement

The bug that this code fixed has been patched in the the latest versions
of bundler and is no longer required.

Gemfile

@@ -6,11 +6,6 @@ git_source(:github) { |repo| "https://github.com/#{repo}.git" }
 branch = ENV.fetch('SOLIDUS_BRANCH', 'master')
 gem 'solidus', github: 'solidusio/solidus', branch: branch
 
-# Needed to help Bundler figure out how to resolve dependencies,
-# otherwise it takes forever to resolve them.
-# See https://github.com/bundler/bundler/issues/6677
-gem 'rails', '>0.a'
-
 # Provides basic authentication functionality for testing parts of your engine
 gem 'solidus_auth_devise'