dillon1000/react

Move 'lighthouse' and 'nodegit' to 'optionalDependencies' (#9675)

**what is the change?:**
See title.

**why make this change?:**
Both these dependencies were causing issues when working on React internally at
FB.

'lighthouse' requires node >=6 and we don't want to folks working on React to
using that version of node.
https://github.com/GoogleChrome/lighthouse/blob/master/package.json#L11

'nodegit' for me throws an error related to libssh2 and it's annoying to make
this work in every OS. See https://github.com/nodegit/nodegit/issues/1266

**test plan:**
`npm/yarn install`
Runs with no errors on my CentOS machine and also on MacOSX laptop.

**issue:**
This is blocking work related to https://github.com/facebook/react/issues/9398