Use Homebrew libyaml if it is available

This checks for Homebrew's yaml to see if it is available and that
`--with-libyaml-dir` hasn't been specified.

Related issue: #379

This was the obvious one to do.  Readline and OpenSSL require a
different treatment.  e.g. We probably don't want openssl 1.0.1
for Ruby < 2.0 and installing/compiling readline for everyone
may not be ideal.