Travis: formula installation

Test several formulas installation as part of the Travis build.

This PR adds `brew install` steps to the Travis build, namely:
- Bio-Formats (last stable release and last develop release with `--devel`)
- Ice 3.3 and 3.4 formulas.

OMERO was another candidate for inclusion as part of this PR but the build step errors while exceeding the 10 minutes timeout enforced by Travis. I suggest keeping the OMERO installation on our CI for now.