Creating a version for Perl 5.18.
Creating a version for Perl 5.18.

* Enable threads by default.
* Add option for DTrace support.
* Make formula keg_only to avoid conflicts with other versions.

Closes #138.

Signed-off-by: Xiyue Deng <4900478725231961caf32165c870e05cea7389d8@gmail.com>

jimhe/homebrew-versions,heuermh/homebrew-versions,Ashton-W/homebrew-versions,UniqMartin/homebrew-versions,coeligena/homebrew-verscustomized,DomT4/homebrew-versions