Replace sed with tr for __strip_duplicates.

The sed command fails on FreeBSD as it joins all of the passed function
names with a 'n' character which results in the functions not being
found. I've used tr to fix the issue as its a bit simpler and does
appear to function similarly between FreeBSD and Ubuntu.
Merge pull request #63 from geoffgarside/bsd-strip-dupes

Fix FreeBSD function detection