Resolve "rack-test >= 0.8.0 requires ruby 2.2.2, makes the build fail on ruby 2.1"
rack-test 0.8.0 added a required_ruby_version of >= 2.2.2, which made the build fail on ruby 2.1. Therefore, pin rack-test to a version < 0.8.0. Once we cease to support ruby 2.1, this should be dropped, again.
Closes #310 (closed)