[RFC] Add nightly build.

[Travis build](https://travis-ci.org/fwalch/bot-ci/jobs/37064879) is currently running. If it works, the release will show up at https://github.com/fwalch/neovim/releases/nightly. Had a buffer overflow on Travis during `make install` (generating helptags) with build type `MinSizeRel`, trying again with default.

Currently, this will update the `nightly` tag to point to the commit hash the tarball was built from. WIP because of that and because I wanted to create a 32-bit nightly as well.

@myitcv would this help with neovim/neovim#1135? It basically uploads a ZIP of everything installed by `make install`.
:+1:

I had originally added the `travis` suffix to indicate that the nightly will not work on end-user systems and is only intended for Travis.

For example, for me it fails with `./nvim: error while loading shared libraries: libtinfo.so.5: cannot open shared object file: No such file or directory`. And while you can execute the compiled `nvim` on any Ubuntu 12.04, it won't find the runtime files by default; you have to export `$VIM` or similar to make it work.

I think I didn't really explain that previously, sorry, but maybe you already knew. It's just that I associated "nightly builds" with some unstable version end-users can execute on their machines, and this is only intended for API clients.

What about adding a note on the release page that this will only work on Travis, and that end-users shouldn't download it, maybe referring them to the install instructions on the wiki?