Make PCL HEAD mandatory on Mavericks

I propose making PCL HEAD mandatory on Mavericks and clang. PCL 1.7.1 does not build with clang and libc++ on Mavericks, but there have been some fixes in head that make this possible. In order to avoid confusion I think it would be best to force the user to use HEAD until the next PCL release rolls out.

Also bump formula to 1.7.1