[WIP] Add coverity and rebuild-deps build types.

## Summary

This PR adds rebuilding of dependencies on Travis CI and pushes to `neovim/deps`. It also adds the coverity trigger from `neovim/neovim` and should remove the need for a `coverity-scan` branch there.

Two changes in `neovim/neovim` are necessary for this: neovim/neovim#1037, neovim/neovim#1033.

## Coverity

Coverity is simply triggered for each build on this repo. Since this should happen only once per week (?) or if something is pushed, I hope that's okay. A commit that adds the encrypted `COVERITY_SCAN_TOKEN` would need to be added to make this work. If I understood correctly, with this it shouldn't be necessary to maintain a separate `coverity-scan` branch in `neovim/neovim` anymore.

## Dependency rebuilding

Also added rebuilding of the dependencies to this PR. If this step is performed periodically, hopefully problems such as the ones related to luarocks ([1](https://github.com/neovim/neovim/issues/1013) [2](https://github.com/neovim/neovim/pull/1023#issuecomment-50985307)) will be noticed more quickly and not only locally, but also on Travis.

This will unfortunately require a [small change](https://github.com/fwalch/neovim/commit/51f2249031ae4ad0dbf4908b9c6c82ff1baa5eb3) to `neovim/neovim`'s travis.sh (neovim/neovim#1037) to change `/opt/neovim-deps` to `/opt/neovim-deps/usr`, or a change to `third-party/CMakeLists.txt` to be able to set `DEPS_INSTALL_DIR` etc. directly. Paths are [hard-coded](https://github.com/neovim/deps/blob/master/bin/luarocks#L3) in some files, so just moving the files around after building doesn't work.

## Helper script for local builds

Added a `local.sh` script to locally test builds more easily (see updated readme). Also testing builds on Travis should now be easier (see fwalch/bot-ci@f9bdabdb0832f4264aa6a84c0a3147cb0b8ba740).

## Final notes

I think it's easier to review this commit by commit. If this is too much for one PR, I'll split it into multiple ones.

Regarding #4: I think that coverity triggering doesn't really belong into `neovim/neovim`. I know that there's already a dependency rebuilding script in `neovim/deps`, but I think it's easier to manage PRs if everything related to "bot builds" is centralized in this repo, and the "output repos" (`neovim/doc`, `neovim/deps`) contain as few non-autogenerated stuff as possible. I also think stuff like this should be run automatically and not rely on someone merging a branch to trigger a build.

Here is a [Travis build](https://travis-ci.org/fwalch/bot-ci/jobs/31692670); output in [fwalch/doc](https://github.com/fwalch/doc) and [fwalch/deps](https://github.com/fwalch/deps). Here's a [neovim build](https://travis-ci.org/fwalch/neovim/builds/31692712) that uses the auto-generated dependencies from `fwalch/deps`. Both builds are using testing branches where I added a commit that changes the repositories from `neovim/*` to `fwalch/*`.

That was quite a lot of text, sorry.. please let me know what you think!

Ping @aktau @jszakmeister @justinmk @ZyX-I