Seems 13.11 was not being used for a while, but 14.27 works.
For some reason, stack was getting confused when I set the magicmime
flag on the command line, but setting it in stack.yaml didn't confuse
it. Ugh.
Unfortunately, the autobuilder is using an old version of OSX, and
cannot use ghc newer than the one in lts-13.11. Newer ghc versions use a
libc symbol that is not available.
Since the main stack.yaml has been updated to a newer lts, and just
substituting in lts-13.11 no longer works due to other dependency
issues, it's simplest to use a separate one for the OSX build.
Hopefully this is temporary, and the autobuilder gets updated.
