[travis] Improvements to main script

- Add README.ci
  Suggestions and comments welcome.
- Use the system compiler to get some boot speedup.
- Build log folding.
- Set NJOBS=2 (very moderate speedup)
- Set language to a defined value.
  OPAM itself recommends C, so we follow suit.
- Remove spurious `.opam`test

  No harm in testing we are in the right opam setting even if the
  cache did exist.

  Anyways, it seems that the previous was spurious, as it was testing
  if ~/coq/.opam did exists. I think the correct command would have
  been:

```shell
[ -e ${HOME}/.opam ] || opam init ...
```

  See the log at https://travis-ci.org/coq/coq/builds/198948812 for an
  example.

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