Upgrade FFI now its safe to do so

Author: JonRowe

Gemfile

@@ -20,8 +20,8 @@ end
 
 gem 'capybara'
 
-# Until 1.13.2 is released due to Rubygems usage
-gem 'ffi', '~> 1.12.0'
+# ~> 1.12 is for Ruby 2.2
+gem 'ffi', '~> 1.12.0 or ~> 1.15.5'
 
 custom_gemfile = File.expand_path('Gemfile-custom', __dir__)
 eval_gemfile custom_gemfile if File.exist?(custom_gemfile)