REDO: Remove leading path separator (:) from GOPATH
REDO: Remove leading path separator (:) from GOPATH

If TM_GOPATH was not set, the resulting GOPATH was starting with
colon (i.e. ":$GOPATH"), which was causing troubles in go1.5

Reapply cherrypick of efa0658a654698577562f1ec2909b9d2f77bfad1 from tg.
Previously attempted in cf4a25062bf453ca1a46a4ad7ef7ff01740f350b,
accidentally overwritten in 26538901bcb6253660bf4fab52dbc336ed984e2c.
Issue identified by tg in https://github.com/syscrusher/golang.tmbundle/issues/17.

syscrusher/golang.tmbundle,tg/golang.tmbundle,tg/golang.tmbundle,syscrusher/golang.tmbundle,syscrusher/golang.tmbundle