Travis: Prepare for OS X builds.

 * Use bot-ci script at to set up prebuilt dependencies.
 * Make CI scripts more consistent.
Merge pull request #1538 from fwalch/multi-os

Travis: Prepare for  OS X builds.
Remove BeOS/Haiku references
Platform cleanup: runtime/autoload/getscript.vim
Platform cleanup: runtime/autoload/netrw.vim
Platform cleanup: runtime/doc/gui.txt
Platform cleanup: runtime/autoload/tar.vim
Platform cleanup: runtime/autoload/vimball.vim