Memory leak and FreeBSD fixes

• There was a leak in the MMDB_open() sub when it was called against a file
  which did not contain any MMDB metadata. Reported by Federico
  G. Schwindt. GitHub issue #36.
• Fixed an error that occurred when passing AI_V4MAPPED to getaddrinfo() on
  FreeBSD. Apparently this macro is defined but doesn't work the way we
  expected it to on that platform.
• Made sure to call freeaddrinfo() when a call to getaddrinfo() fails but
  still allocated memory.
• Fixed a segfault in the tests that occurred on FreeBSD if we passed a NULL
  value to freeaddrinfo().
• Added a missing step to the README.md file for installing from our GitHub
  repository. Patch by Yasith Fernando.
• Added instructions for installing via Homebrew. Patch by Yasith Fernando.