Partial thread-safety fix (lib/airbrake/rails/middleware.rb is still bro...

...ken)

The current version of the airbrake gem is no longer thread-safe (since dec07d249eeaec8384fbf0c4d39e6642e8592fa1). This change partially fixes the issue.

lib/airbrake/rails/middleware.rb is, however, still broken as I am not exactly sure from which context the methods accessing `@env` are called (and thus how to alternatively supply `env` without using a virtually global attribute).