Remove old mri platform entries from our Gemfile

Since we now only support Ruby 2.2+, we can safely remove the old mri
entries from the Gemfile.

This also allows us not to lock to a specific bundler version, since
:mri_22 was defined only on 1.7.11.

Closes #19611.

Gemfile

@@ -63,18 +63,11 @@ group :test do
   # FIX: Our test suite isn't ready to run in random order yet
   gem 'minitest', '< 5.3.4'
 
-  platforms :mri_19 do
-    gem 'ruby-prof', '~> 0.11.2'
-  end
-
-  platforms :mri_21, :mri_22 do
+  platforms :mri do
     gem 'stackprof'
+    # gem 'byebug'
   end
 
-  # platforms :mri do
-  #   gem 'byebug'
-  # end
-
   gem 'benchmark-ips'
 end
 

Gemfile.lock

@@ -185,7 +185,6 @@ GEM
       redis (~> 3.0)
       resque (~> 1.25)
       rufus-scheduler (~> 3.0)
-    ruby-prof (0.11.3)
     rufus-scheduler (3.0.9)
       tzinfo
     sdoc (0.4.1)
@@ -276,7 +275,6 @@ DEPENDENCIES
   redcarpet (~> 3.2.2)
   resque
   resque-scheduler
-  ruby-prof (~> 0.11.2)
   sdoc (~> 0.4.0)
   sequel
   sidekiq