Optimize execution duration for rspec specs

* Use in memory sqlite database for test environment
* Monkey patch BCrypt default cost factor
* Clean spec/spec_helper.rb

spec/support/decrease_bcrypt_cost_factor.rb

@@ -0,0 +1,8 @@
+module BCrypt
+  class Engine
+    [:DEFAULT_COST, :MIN_COST].each do |sym|
+      remove_const  sym
+      const_set     sym, 1
+    end
+  end
+end