Fix lambda space incompatible with Ruby 1.9

Fails install on Ruby 1.9.3, because a syntax error in lambda expression:

As explained in this post, this work fine in Ruby 2.0, but fails in Ruby 1.9:
http://ruby-journal.com/becareful-with-space-in-lambda-hash-rocket-syntax-between-ruby-1-dot-9-and-2-dot-0/