v0.5.1

Packaging and test-suite fixes. Nothing a reader will notice: of the three
changes here, two are to files excluded from the published crate, and the
third is internal.

Fixed

• The Homebrew formula had no url and no sha256. Not stale — absent,
  since it was written, through four releases. brew install had nothing but
  --HEAD to fetch, and the version its own test block asserts against did
  not exist. It now points at the tag's source tarball, which is what the
  formula builds from; the checksums.txt on a release describes the prebuilt
  archives it never downloads.
• The configuration tests read whoever was running them.
  Config::load(.., None, ..) stubbed the environment but not the filesystem,
  so locate reached for the real default path and "nothing configured" meant
  "nothing configured, unless this machine has a file". Three tests asserting
  the defaults were asserting the machine had no configuration. They passed for
  as long as they did because hardly anyone had one — and then the theme picker
  started writing it, so trying the headline feature of 0.5.0 turned the suite
  red for reasons unconnected to anything the contributor touched. locate is
  handed the location now, which is what the module header always claimed.

Changed

• tests/docs.rs checks the Homebrew formula, and the smoke job checks that a
  configuration file in the usual place is read — the one line no unit test can
  see, since it reaches through dirs. Both were confirmed by breaking them.