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

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.

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 to `neovim/neovim`'s travis.sh 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.

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).

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/builds/31559210); output in [fwalch/doc](https://github.com/fwalch/doc) and [fwalch/deps](https://github.com/fwalch/deps).

That was quite a lot of text, sorry.. waiting for your comments! :-)

Ping @aktau @jszakmeister @justinmk @ZyX-I