Fix idempotence of symlink creation in Rakefile spec_prep task

The presence of a symlink was being tested with File.exists?, but this was not
reliable in my own testing.  I changed it to File.symlink?, which I hope
reflects the intent.
Merge pull request #34 from L2G/rake-symlink-idem

Fix idempotence of symlink creation in Rakefile spec_prep task