Fix WatchmanFileScanner for Ruby < 1.9.2 compatibility

`File.realpath` appeared in 1.9.2 (I believe), so swap it out for
Pathname instead, which should work everywhere.

Note that the call as it existed was not optimal anyway, as the call to
`File.expand_path` was redundant.

Signed-off-by: Wincent Colaiuta <win@wincent.com>