Update running_a_rails_app.md

Since c9 in telling user to MySQL in rails app so it's better to create database.yml settings for MySQL by default by passing "-d DatabaseName", updated 

rails new example

to

rails new example -d mysql
Update running_a_rails_app.md

Now no need to add mysql2 gem in gemfile so removing "Install mysql2 adapter by typing in terminal: gem install mysql2" line.
Merge pull request #83 from ahmadhasankhan/master

Making MySQL Default database.