[RFC] Rebuild dependencies (includes Busted v2 update)

Updated the dependencies on Travis using the work-in-progress `rebuild-deps` bot-ci script. Here's the build that updated busted: https://travis-ci.org/fwalch/bot-ci/builds/35041447. I squashed the 32/64 commits after Marvim committed them, that's why this PR has only 2 commits.

This PR includes moving the dependencies into `64` and `32` subfolders (see also neovim/neovim#1037) to allow the bot-ci script to check out subtrees of the `deps` repo and run 64 and 32 bit builds in parallel (similar to how clang report, translation report, ... are run in parallel on bot-ci). Also includes the update to Busted v2 (neovim/neovim#1098).

A successful Neovim build based on current master and using the updated dependencies from this PR as well as neovim/neovim#1037 and neovim/neovim#1098 is here: https://travis-ci.org/fwalch/neovim/builds/35042442.

Therefore, the following should be merged more or less simultaneously:

* this PR
* neovim/neovim#1037
* neovim/neovim#1098

Ping @justinmk.