AURORA-607:release script is broken

Updates release script to promote a voted on rc to an official release.

Testing Done:
./build-support/release/release -r 2

I have not tested the actual promotion of the rc to release, as this is the first official ASF release of Apache Aurora I saved that for Kevin to do

Steps to publish the 0.5.0 release with this script:
* git checkout 0.5.0-rc2
* apply this patch
* comment out dirty tree exit condition, line 89 of ./build-support/release/release
* test with: ./build-support/release/release -r 2
* rm -rf dist
* publish with: ./build-support/release/release -r 2 -p

Bugs closed: aurora-607

Reviewed at https://reviews.apache.org/r/24173/