Only add missing directories to PATH
Only add missing directories to PATH

Only add the directories which aren't already present in PATH and
append the first element of GOPATH instead of prepend it.

This prevents failures issues caused by miss-ordered entries in
PATH fixing #266.

alecthomas/gometalinter,dnephin/gometalinter,alecthomas/gometalinter,alecthomas/gometalinter,dnephin/gometalinter,dnephin/gometalinter