drop rmagick, fixes #4594

This breaks the screenshot tool.

RMagick no longer compiles on recent ImageMagicks that have HDRI
support enabled.

Gemfile

@@ -168,10 +168,6 @@ group :production do # we don't install these on travis to speed up test runs
 end
 
 group :development do
-  # Comparison images
-
-  gem 'rmagick', '2.13.2', :require => false
-
   # Automatic test runs
   gem 'guard-cucumber', '1.4.1'
   gem 'guard-rspec',    '4.2.4'

Gemfile.lock

@@ -350,7 +350,6 @@ GEM
     redis-namespace (1.4.1)
       redis (~> 3.0.4)
     remotipart (1.2.1)
-    rmagick (2.13.2)
     roxml (3.1.6)
       activesupport (>= 2.3.0)
       nokogiri (>= 1.3.3)
@@ -514,7 +513,6 @@ DEPENDENCIES
   rb-inotify (= 0.9.3)
   redcarpet (= 3.0.0)
   remotipart (= 1.2.1)
-  rmagick (= 2.13.2)
   roxml (= 3.1.6)
   rspec-instafail (= 0.2.4)
   rspec-rails (= 2.14.1)