switch to debugger gem for 1.9

It appears that ruby-debug is mostly no longer maintained. debugger is a fork
that appears to get regular updates, for now.

Change-Id: I026dabbede9404c6fc6a505e8ff2dca19d8961d5
Reviewed-on: https://gerrit.instructure.com/10710
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brian Palmer <brianp@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>

Gemfile

@@ -97,9 +97,7 @@ group :development do
   gem 'coffee-script-source',  '1.3.1' #pinned so everyone's compiled output matches
   gem 'parallel',     '0.5.16'
   if ONE_NINE
-    # ruby-debug is broken in 1.9.3:
-    # http://blog.wyeworks.com/2011/11/1/ruby-1-9-3-and-ruby-debug
-    # gem 'ruby-debug19', '0.11.6'
+    gem 'debugger',     '1.1.3'
   else
     gem 'ruby-debug',   '0.10.4'
   end

config/environments/development.rb

@@ -41,6 +41,8 @@ Dir[File.dirname(__FILE__) + "/" + File.basename(__FILE__, ".rb") + "-*.rb"].eac
 
 # allow debugging only in development environment by default
 # ruby-debug is currently broken in 1.9.3
-if RUBY_VERSION < "1.9.3"
+if RUBY_VERSION < "1.9."
   require "ruby-debug"
+else
+  require "debugger"
 end