BUGFIX: Homebrew chg, 3 args for checksum mismatch

Homebrew changed underneath us, requiring 3 arguments to
`ChecksumMismatchError.new`.  This is causing total failure
on Travis, and probably a number of issues will be filed
until we merge and cut a bugfix release.
Merge pull request #3044 from rolandwalker/checksum_three_arg

BUGFIX: Homebrew chg, 3 args for checksum mismatch