Update minimum version of image_optim gem

Keep us up-to-date with the latest and greatest.

Change-Id: Ifd19842ae064987690eb5538e4d8af24401d4a0a
Reviewed-on: http://gerrit.causes.com/46996
Tested-by: jenkins <jenkins@brigade.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>

Author: sds

overcommit.gemspec

@@ -28,7 +28,7 @@ Gem::Specification.new do |s|
 
   s.add_dependency             'childprocess', '~> 0.5'
 
-  s.add_development_dependency 'image_optim', '~> 0.18.0'
+  s.add_development_dependency 'image_optim', '~> 0.20.0'
   s.add_development_dependency 'rspec', '~> 3.0'
   s.add_development_dependency 'travis', '~> 1.7'
 end