Use OSX package hosted in github for OSX build

[bcardiff]

Currently, we rely on homebrew formula. When publishing a new release the previous version information is not explicit for OSX as in Linux.

Using official .pkg release will leave the CI in a more controlled/reproducible state.
It will also helo to evolve/test changes in the packaging without dealing with brew formula state.

Optionally a nightly/weekly can be added to compile crystal from sources using the brew formula to ensure that things are working smoothly on when using that package manager.

[bcardiff]

The OSX is broken because libyaml in brew was update from 0.2.1 to 0.2.2.

• https://circleci.com/gh/crystal-lang/crystal/20015

We could decide better when to fix the spec to tolerate 0.2.2 changes if we wouldn't depend to much on brew for the CI.