Add installation script to .travis.yml

Create travis/script.sh adapted from OMERO-homebrew-install.sh in
https://github.com/openmicroscopy/openmicroscopy.git
This script installs Bio-Formats and OMERO using Homebrew and starts the
server after an initial set-up.
Drop brew doctor step and fix PATH prepending

Xcode may be outdated on the Travis node which would fail the build (see
https://travis-ci.org/sbesson/homebrew-alt/builds/10315498)
Tap ome/alt and merge opened PRs
Fix local PRs merging in the tap
Add global read-only token to  merge homebrew-alt PRs locally
Drop Homebrew Python installation and pre-install scc and genshi