Use knapsack to parallelize tests

...in order to reduce TravisCI runtimes

spec/spec_helper.rb

@@ -1,4 +1,5 @@
 # This file is copied to spec/ when you run 'rails generate rspec:install'
+require 'knapsack'
 require 'simplecov'
 
 if ENV['TRAVIS']
@@ -102,3 +103,4 @@ RSpec.configure do |config|
 end
 
 OmniAuth.config.test_mode = true
+Knapsack::Adapters::RSpecAdapter.bind