Lazy data for GRIB.

> ... why we need to do it in CI.

Where else would you test it? It does no good to leave issues that break user installations until a release is made.

> ... this has been discussed ...

See #394 (OK, not too much discussion there, actually...)

> 4. Any more?

How about just test both? Add an environment variable `env: TEST_USER_INSTALL=yes/no`, and have the Travis script do one or the other based on that value. Sure, it means doubling the number of tests, but they're run in parallel on Travis, so it's not a big deal.