Have homebrew auto-detect dartsim dependencies.

[egordon]

Referencing PR #570 .

Currently, in the Travis build for OS X Mojave, the dependencies for dartsim need to be listed manually in the Brewfile.

A mistake in this file has already caused at least one bug.

We should investigate as to why Travis and Homebrew do not pull the dependencies automatically. In the worst case, we should be able to use brew deps and some text parsing to do it ourselves.