fix gopath

[commojun]

fixed gopath.

Since the last slash in GOPATH is unnecessary, the slash in PATH was doubled as in the example below.

$ echo $GOPATH
/home/commojun/go/
$ echo $PATH
... :/home/commojun/go//bin:/usr/local/go/bin: ...

This isn't a fatal problem in a normal shell, but Emacs wasn't able to correctly recognize duplicate slashes when using this PATH.

[crramirez]

Thank you for the contribution