add go-nat-pmp to manual install list

.travis.yml

@@ -3,6 +3,7 @@ before_install:
   - sudo apt-get update -qq
   - sudo apt-get install -yqq libgmp3-dev qtbase5-private-dev qtdeclarative5-private-dev libqt5opengl5-dev libreadline6-dev
 install:
+  - go get github.com/jackpal/go-nat-pmp
   - go get code.google.com/p/go.tools/cmd/goimports
   - go get github.com/golang/lint/golint
   - go get code.google.com/p/go.tools/cmd/vet