String#scrub requires Ruby 2.1

Hi, thanks for the great work, GitLab!

Through the Bitnami VM I came across a possible documentation issue. Since f6609c98 search requires String#scrub, which appeared in Ruby 2.1.*. Thus, either the readme should be updated not to include Ruby (MRI) 2.0 support a fall-back.

Under Ruby 2.0 search fails due to the missing method.

Cheers

D