Make rake a runtime dependency

RSpec has it as a dependency of rspec-support.

Fixes #70

Author: elia

opal-rspec.gemspec

@@ -26,9 +26,9 @@ Gem::Specification.new do |spec|
 
   spec.add_dependency 'opal', ['>= 0.11', '< 0.12']
   spec.add_dependency 'opal-sprockets'
+  spec.add_dependency 'rake', '~> 12.0'
 
-  spec.add_development_dependency "bundler", "~> 1.15"
-  spec.add_development_dependency "rake", "~> 12.0"
+  spec.add_development_dependency 'bundler', '~> 1.15'
   spec.add_development_dependency 'yard'
   spec.add_development_dependency 'chromedriver-helper'
   spec.add_development_dependency 'selenium-webdriver'