[RFC] Build nightly based on latest successful Travis build.

Looks at the last 50 builds (from pushes, not PR builds) on `neovim/neovim` and searches the latest successful one on `master`, then builds the nightly from that.

I tested it with my Neovim fork. It is at current `neovim/neovim` master (65942d3a8d84510fcee2dd1c6306a5af13296c84), but its last successful Travis build points at 6e268cd0d40a3652a68b486bdbb421d39295ab48. This is the version used to [create](https://travis-ci.org/fwalch/bot-ci/jobs/39031280) the [nightly](https://github.com/fwalch/neovim/releases/nightly):

```
Using commit 6e268cd0d40a3652a68b486bdbb421d39295ab48 from latest successful Travis build on fwalch/neovim:master.
nightly tag does not point to 6e268cd0d40a3652a68b486bdbb421d39295ab48, continuing.
```