stackage-setup: use nightly or lts version numbers for s3 distribution archives

[DanBurton]

This means:

• making sure that they build against 7.10, so that the upcoming nightlies can be used
• making sure that all included binaries are included in the snapshot

The goal is an unambiguous way to refer to precise versions of all these stackage-* binaries as a bundle. Stackage nightly and/or lts snapshots are a fantastic way of accomplishing this.

[DanBurton]

Tomorrow's nightly (2015-05-07) should be suitable for this, assuming all goes well.

n.b. July 1 (earliest release of LTS 3) makes for a nice stabilization goal date.

[DanBurton]

Although we are going to consolidate the executables back into one repo, I still think it useful to express the bundle's version in terms of which snapshot it was built in. This expresses not only the version of the code for the executables, but also the versions for all dependencies they were built against as well.