Changed using Flux: Tracker to using Flux.Tracker

[cpfiffer]

This is possibly a pull request to fix #583, which appears to be a deprecation problem.

Currently we import Flux.Tracker like this:

using Flux: Tracker

When according to this it should be:

using Flux.Tracker

I wanted to open up a pull request here just to make sure I'm correct here. Thoughts?

[mohamed82008]

This seems irrelevant to the linked Turing issue as warnings don't break compilation. Also the v0.7.0 warning for using A: B when B is a module seems to be a false threat as both work on Julia v1.0 and no breaking changes are happening in 1.x unless extremely necessary. But this seems like a harmless change to me, so I don't mind it.

[cpfiffer]

It struck me as odd that I haven't seen this error on 1.0+. The patch notes seemed to indicate that this was a breaking change in 0.7.

[mohamed82008]

They may have missed it 😝. Anyways, it's too late now to change it and 2.0 is quite far.