I receive this warning when installing LLVM on OS X:

==> Warning: There is no checksum on file to fetch python@2.7 safely.
  Fetch anyway? [y/N] ^CTraceback (most recent call last):

Spack is trying to install Python 2.7 (not 2.7.x). When I answer "yes", the download succeeds, but the build ultimately fails for unrelated reasons. Spack should not be installing this version; it should install 2.7.11 instead.

I assume this might be because LLVM requires depends_on('python@2.7:2.8'), and Spack resolves this to Python 2.7 instead of Python 2.7.11?