Install go automatically with SCION dependencies

[mlegner]

In the tutorials for Ubuntu 16.04 and RaspberryPI, we explicitly state which version of Go the user should install.
However, it is possible to have SCION install go together with other dependencies via GO_INSTALL=true ./env/deps (see also https://github.com/netsec-ethz/scion-coord/blob/master/vagrant/Vagrantfile).
I believe, that this will be easier to maintain, as the Go version may change frequently in the future (we may soon cherrypick the update to go1.9 into the scionlab branch).
In addition, we could even offer some simple install script (similar to the one in the Vagrantfile linked above), which does all the necessary steps for the user.