Add explicit require for faraday

--- a/spec/spec_helper.rb
+++ b/spec/spec_helper.rb
@@ -16,6 +16,7 @@
 #
 # See http://rubydoc.info/gems/rspec-core/RSpec/Core/Configuration
 
+require 'faraday'
 require 'simplecov'
 require 'coveralls'
 require 'webmock/rspec'