Tell the user if they have a snapshot build.

Snapshot builds are convenient for people to test bugfixes against
but there's an inherent risk that people will use them, which is
very, very bad.  This commit looks for SNAPSHOT versions and
heroically warns the user.

This will also pick up stray snapshot versions at release time.
Merge pull request #445 from simpsonjulian/snapshot-warning

Tell the user if they have a snapshot build.