have create_milestone rake task create release on the current branch

[blairconrad]

Right now, all releases will be created based on master.
This caused a problem when I created release 1.25.3 (#512).

If the create_release command were told to use the current branch (git rev-parse --abbrev-ref HEAD) instead, we'd have a better chance of doing the right thing.

[adamralph]

Sounds good.

[blairconrad]

This no longer applies after removing the issue/release/milestone-manipulating build targets while fixing #645.