Fix command typo and updating ruby version

They typo was that the word 'ruby' was extra.
I've just tested it right now.
Link related: http://bl.ocks.org/javisantana/81539b0d45deb1714671

README.md

@@ -500,7 +500,7 @@ sudo bundle install
 rvm use 1.9.3@cartodb --default
 
 # If you are using rbenv simply run:
-rbenv global ruby 1.9.3-p547
+rbenv local 1.9.3-p551
 bundle install
 
 # Configure the application constants