Rename spec/helper.rb to spec/spec_helper.rb

* This naming is more inline with common convention
Replace single letter variable with longer variable name
Renamed instance variable to only use a single leading underscore

* The single leading underscore, along with the namespace prefix
  should be enough to ensure that it does not collide with a regular
  object's own instance variable state.
Change Rakefile to have require statements before task setup
Remove unnecessary require statement

* The main memoizable.rb file requires thread_safe now
Change gemspec to match other gems
Add minimum spec coverage threshold
Add more rubies to test against on travis