Cleanup Ruby 2.1 warnings on File.exists?

Mostly cherry-picks from master:

    commit d48222d22ef576c1b429ec2321c712bc221698d7
    Author: laurocaetano
    Date:   Fri Nov 1 18:15:53 2013 -0200

    commit 49062c12edc4b5397ea3bc6680eb6a13bdec2164
    Author: Rashmi Yadav
    Date:   Fri Nov 1 18:07:27 2013 +0100

    commit c10a78124c1c32f8ce3a92888863b9b258efcc06
    Author: Arun Agrawal
    Date:   Fri Nov 1 10:39:06 2013 +0100

    commit 816126862e040b787cf11c8143f326798803f67c
    Author: Arun Agrawal
    Date:   Fri Nov 1 09:49:57 2013 +0100

    commit 4d4ff531b8807ee88a3fc46875c7e76f613956fb
    Author: Aaron Patterson
    Date:   Thu Oct 31 11:47:51 2013 -0700