Rename releases.csv as elixir.csv, create erlang.csv

The format for `erlang.csv` is fairly self-explanitory:
```
OTPRelease,ERTSVersion,WinInstall32bitURL,WinInstall64bitURL
```
We really only need the latest release, but this might be a good place to document which Erlang releases work with Elixir.  We could even add another argument for the latest Elixir version that runs on that OTP release.

As for `elixir.csv`, we'll remove the other entries once v0.14.3 comes out, since the installer might not like an empty file.