Use stackprof to test the lack of object allocations

[dylanahsmith]

Problem

I was getting a bunch of warnings when running the tests that were caused by the Spy gem

/Users/dylansmith/.gem/ruby/2.3.3/gems/spy-0.4.1/lib/spy/subroutine.rb:55: warning: method redefined; discarding old any?
/Users/dylansmith/.gem/ruby/2.3.3/gems/spy-0.4.1/lib/spy/subroutine.rb:55: warning: method redefined; discarding old empty?
/Users/dylansmith/.gem/ruby/2.3.3/gems/spy-0.4.1/lib/spy/subroutine.rb:204: warning: instance variable @plan not initialized
/Users/dylansmith/.gem/ruby/2.3.3/gems/spy-0.4.1/lib/spy/subroutine.rb:72: warning: method redefined; discarding old any?
/Users/dylansmith/.gem/ruby/2.3.3/gems/spy-0.4.1/lib/spy/subroutine.rb:228: warning: previous definition of any? was here
/Users/dylansmith/.gem/ruby/2.3.3/gems/spy-0.4.1/lib/spy/subroutine.rb:72: warning: method redefined; discarding old empty?
/Users/dylansmith/.gem/ruby/2.3.3/gems/spy-0.4.1/lib/spy/subroutine.rb:228: warning: previous definition of empty? was here

Solution

We were just using the spy gem in a single test, where we were testing the implementation instead of what we actually cared about, avoiding object allocations. I changed the test to use stackprof to count object allocations so we could assert that none were made.

This also fixes the Gemfile so that we can use stackprof to profile any any of the support MRI versions. Previously it would only install stackprof for MRI ruby 2.1, which is now the minimum ruby version we support.

[jasonhl]

Much better, thank you.