Remove --devel flag for go Formula update stable to 1.1.

Also require the use of the correct clang compiler 
by default. This change was needed when using 
--devel and now needed in stable.

A new caveat has also been added explaining how a 
$GOPATH must be set and cannot be the same as 
$GOROOT From the 1.1 release notes listed below.
http://golang.org/doc/go1.1#gocmd

Added a new --keep-headers flag so you can run the 
system tests of go after being installed or 
by default a simple hello world.