Lock optparse < 0.7

Workaround a regression in ruby-head: ruby/optparse#93

Author: byroot

Gemfile

@@ -10,6 +10,7 @@ group :development do
   gem "test-unit"
   gem "test-unit-ruby-core"
   gem "all_images", "~> 0" unless RUBY_PLATFORM =~ /java/
+  gem "optparse", "< 0.7"
 
   if ENV['BENCHMARK']
     gem "benchmark-ips"