Relax image_optim dependency

Make it depend on the last minor version, rather than revision.

Change-Id: Ief6540676eb1aad58ff45d6c2354f68bb8f9e88e
Reviewed-on: http://gerrit.causes.com/37008
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', '0.13.0'
+  s.add_development_dependency 'image_optim', '~> 0.13.0'
 end