conditionally check for $GOPATH/bin before moving it

I was able to get the following to work and deploy to heroku:

```
go get github.com/dougnukem/revel-helloworld
cd $GOPATH/src/github.com/dougnukem/revel-helloworld
heroku create
heroku config:set BUILDPACK_URL=https://github.com/dougnukem/heroku-buildpack-go-revel.git
git push heroku master
heroku open

```