fix undefined method namespace error by not requrire in Gemfile

I have following errors when I was using this gem:
>undefined method `namespace' for main:Object (NoMethodError)

And found that I am requiring the wrong repos, and not require in Gemfile so that I can by-pass this namespace error. 
> gem 'capistrano-maintenance', github: "capistrano/maintenance", require: false 

So just update README, in case other follower may confuse when using this gem.