Fix wrong command on homepage

There is an error on homepage. You indicate to run:

    go install -u github.com/hybridgroup/gobot

But it should be:

    go get -u github.com/hybridgroup/gobot

As indicated in the [Getting started doc](http://gobot.io/documentation/getting-started/).

Fixes https://github.com/hybridgroup/gobot/issues/117#issuecomment-57947138