should not install cap version above 2.x

When installed didi will fail on cap version 3.x
```
/Users/robbiebardijn/.rvm/rubies/ruby-2.1.0/lib/ruby/site_ruby/2.1.0/rubygems/core_ext/kernel_require.rb:126:in `require': cannot load such file -- capistrano/cli (LoadError)
	from /Users/robbiebardijn/.rvm/rubies/ruby-2.1.0/lib/ruby/site_ruby/2.1.0/rubygems/core_ext/kernel_require.rb:126:in `require'
	from /Users/robbiebardijn/.rvm/gems/ruby-2.1.0/gems/capistrano-didi-0.4.4/bin/didi:3:in `<top (required)>'
	from /Users/robbiebardijn/.rvm/gems/ruby-2.1.0/bin/didi:23:in `load'
	from /Users/robbiebardijn/.rvm/gems/ruby-2.1.0/bin/didi:23:in `<main>'
	from /Users/robbiebardijn/.rvm/gems/ruby-2.1.0/bin/ruby_executable_hooks:15:in `eval'
	from /Users/robbiebardijn/.rvm/gems/ruby-2.1.0/bin/ruby_executable_hooks:15:in `<main>'
```