Fix broken_symlink? helper on JRuby

JRuby's implementation of `File.exist?` (JRuby 1.7.13 at time of
writing) returns `true` for broken symlinks instead of `false` like MRI.

Fix this by using `File.size?` instead, which has the correct behavior
on both platforms.

Change-Id: I50a7ed3a9c2b80553e08a0a1d06e955c26f4594e
Reviewed-on: http://gerrit.causes.com/42401
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>