Update Go install instructions

[damienstanton]

Using a modern version of Go (1.15.2 in my testing), the instructions are simplified to the point where I would not call it "harder". GOPATH and GOBIN are no longer required, and GO111MODULE is auto by default. Further, you don't need to separately install bitcomplete, and the installation steps aren't different for Windows anymore.

This patch fixes up the installation instructions to represent a modern Go environment.

[chriswalz]

I'm not sure what you mean by "Further, you don't need to separately install bitcomplete" are you saying Go Get will install both binaries? I'm not sure this is the case.

I checked on my Ubuntu box and my Go 1.15 didn't have a GO111MODULE set to anything so I'm not convinced it' will be consistent across machines. Regardless, I like the idea but I think this needs further investigation

[damienstanton]

Thanks for the reply! My understanding from the tracking issue for full GOPATH deprecation was that the GO111MODULE env var itself should have been deprecated at Go 1.13 (but I might be misreading the thread and history there).

w.r.t the extra binary, I'll try on my Ubuntu box as well to see if I get consistent results. I noticed on my macOS machine that I only ran the single go get command, as in the Windows install instructions and everything worked fine. So what I meant was that it didn't seem clear why the source install mentioned the need to install bitcomplete separately for macOS/Linux.