rack-test >= 0.8.0 requires ruby 2.2.2, makes the build fail on ruby 2.1
See this failed build:
In Gemfile: schleuder was resolved to 3.2.1, which depends on sinatra-contrib was resolved to 1.4.7, which depends on rack-test ERROR: Job failed: exit code 1
This is documented in the release notes of rack-test 0.8.0, and makes the build on ruby 2.1 currently fail.
We should either pull in an older release, or drop ruby 2.1.