Remove "PHP 7 is not supported" from install documentation

Summary:
Ref T9640. This is no longer true.

I'm assuming PHP7 vs 7.1 issues won't be common (everyone on the cutting edge is probably on 7.1, and Ubuntu 16 is on 7.1) but we could make this more granular in the future.

Test Plan: Careful reading.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9640

Differential Revision: https://secure.phabricator.com/D17202