Gemfile maintenance

This patch provides the following changes:

* Remove unnecessary Rails dependency
* Fix Git insecure data fetching warning

Gemfile

@@ -1,11 +1,9 @@
 source 'https://rubygems.org'
 
 branch = ENV.fetch('SOLIDUS_BRANCH', 'master')
-gem 'solidus', github: 'solidusio/solidus', branch: branch
+gem 'solidus', git: '[email protected]:solidusio/solidus.git', branch: branch
 gem 'solidus_auth_devise'
 
-gem 'rails', '~> 5.1', '>= 5.1.6'
-
 if branch < 'v2.5'
   gem 'factory_bot', '4.10.0'
 else