Tests: Allow for -PRE in stable release numbers.

This means we can release `v1.2.0-PRE1` without our test cases going
silly. This release will read `v1.2` spec files (since its version
number is greater than the spec_version), but we're not necessarily
calling it a release candidate.

Internally `ckan.dll` and friends don't care about the version number at
all, except to compare it as a `Version` object against the
`spec_version` in our metadata. We could have `cheese sandwich` as a
version and the core would accept that as being okay. ;)
Merge pull request #553 from pjf/version_tests

Tests: Allow for -PRE in stable release numbers.