Fix inefficient assert pattern in Rack::Lint (rack#1724)

* Use `raise "message" unless condition` instead of assert in Rack::Lint

* The existing #assert is very inefficient because it computes the error
  message string even if no error is raised.

* Fixes rack#1723

* Deprecate Rack::Lint::Assertion#assert

* No need to include the Assertion module anymore in lint.rb

Author: eregon