Installation instructions shouldn't mention python 2.7 anymore

[agjohnson]

RTD runs on python 3! Drop the notice around using python 2.7 only.

Docs here:
http://docs.readthedocs.io/en/latest/install.html

[stsewd]

I was trying to change that on this PR
#3631 (comment)

[irkartik]

@agjohnson @stsewd @humitos installation docs for Readthedocs is missing out so much of information and in my opinion should be updated.
Like, I am stuck up with the version of elasticsearch to be used, tests always fail for python 36 (now I know because I was using py27 virtualenv). If there's a way I can contribute updating the docs, I would love to do so.

[stsewd]

@rajujha373 this PR #3631 aims to do that, can you take a look and tell if that covers your issues?. Also about elasticsearch, there is a discussion about dropping it from the installation guide for now, since in more cases isn't necessary for start to contributing.

[stsewd]

Resolved in #3817