fix rails 4 Gemfile.lock under bundler 1.7

bundler 1.7 @sources no longer responds to include?. Conveniently,
the method we're overriding no longer calls include?

Change-Id: I5362b28a6819a4de9bc1e5de25b4753a8d77d9e1
Reviewed-on: https://gerrit.instructure.com/41510
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>

Gemfile.d/_before.rb

@@ -57,7 +57,7 @@ unless CANVAS_RAILS3
 
   Bundler::Dsl.class_eval do
     def to_definition(lockfile, unlock)
-      @sources << @rubygems_source unless @sources.include?(@rubygems_source)
+      @sources << @rubygems_source if @sources.respond_to?(:include?) && !@sources.include?(@rubygems_source)
       Definition.new(Bundler.default_lockfile, @dependencies, @sources, unlock, @ruby_version)
     end
   end