added rake to the gemfile as it is needed by travis

Author: Enrique Comba Riepenhausen

Diff for: Gemfile

@@ -1,9 +1,10 @@
 source :rubygems
 
 group :development do
-  gem 'rspec'
   gem 'cucumber'
-  gem 'robot-vim'
+  gem 'rake'
   gem 'relish', git: 'https://github.com/mattwynne/relish.git'
+  gem 'robot-vim'
+  gem 'rspec'
 end