Check that METAD + TARGET + GRID_SPARSE works correctly.

Solves a bug with METAD + TARGET + GRID_SPARSE
See also #467

cc: @gtribello

It looks like the change was implemented separately in 2.4 and 2.5

should be not important. In particular, when invsigma was <10-12
sigma was set as equal to invsigma. Now it is set to zero,
which I think is the expected behavior.

When conda build fails, the travis job should return an error.

@tonigi