Update image_optim development dependency

Change-Id: I01b263713ad1211105e4eab118dbac5da2dacec0
Reviewed-on: http://gerrit.causes.com/36005
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>

Author: Shane da Silva

overcommit.gemspec

@@ -29,5 +29,5 @@ Gem::Specification.new do |s|
   s.add_dependency             'childprocess', '>= 0.5.1'
 
   s.add_development_dependency 'rspec', '2.14.1'
-  s.add_development_dependency 'image_optim', '0.11.1'
+  s.add_development_dependency 'image_optim', '0.12.0'
 end