Gemfile: Remove pry out of test group

Travis fails to install debugger, which regularly has to be updated
for new Ruby versions. Travis told us to remove debugger from the test
group, which not only saves installing it when running tests on CI,
but also should make debugger not break our CI runs in the future.

The only downside I see is, that running pry from tests might require
loading pry before using it, so 'binding.pry' becomes
'require "pry"; binding.pry'.

OpenProject Issue: https://www.openproject.org/issues/1262