Raise a RuntimeError if ENV["HOME"] is unset.

It'll alternately allow HOMEDRIVE and HOMEPATH (presumably for cygwin); the
exception does mention this too.
Ensure sure HOME points to a directory
Merge pull request #89 from njvack/raise-if-no-home

Raise an error if requiring rbreadline without ENV["HOME"]

Signed-off-by: Mark Somerville <mark@scottishclimbs.com>