14.04 support

[kevinquinnyo]

So I finally had some time to work on this. I'm not sure what your thoughts are on this but since upstream mcrouter has started adding FOLLY_COMMIT files, this is all much easier.

This PR is an attempt to accomplish two things:

1. Add support for 14.04 without breaking 12.04 support or making too many changes
2. Try to automate the builds further by pulling down the Mcrouter version and commit hash automatically. This is so that the builds could be setup via cron if desired, without having to manually add the newest SHA hash.

Currently, there is a problem upstream. The 0.15.0 (newest release as of now) does not contain the FOLLY_COMMIT file. The correct SHA with that file is actually facebook/mcrouter@bbc2a52

So I would suggest that we wait for their next release and do some testing then when considering merging this in or not.

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[bmorton]

Thanks for doing this! I probably won't have too much time until I get through the holidays a bit, but I'll get back to you on this as soon as I have a chance to dig in.