travis: improve preparation on OS X
travis: improve preparation on OS X

Reset core Homebrew repository to a defined state and wipe out all
pre-installed taps (caskroom/cask, homebrew/versions) and formulae (to
avoid undesirable interaction with the builds later on). Afterwards,
update the core Homebrew repository, symlink the homebrew/science tap
into the pre-installed Homebrew, and adjust environment accordingly.

heuermh/homebrew-science,philippbayer/homebrew-science,pditommaso/homebrew-science,ahundt/homebrew-science,joshbode/homebrew-science,ge11232002/homebrew-science,aeantipov/homebrew-science,kozo2/homebrew-science,pmelsted/homebrew-science,andrewjpage/homebrew-science,parcollet/homebrew-science,goxberry/homebrew-science,dpo/homebrew-science,quantumsteve/homebrew-science,maplesond/homebrew-science,DabeDotCom/homebrew-science