update CI list about helix keyboards

* exclude 'helix/rev3_4rows' from CI
  Since helix/rev3_4rows is almost the same as helix/rev3_5rows, there is no point in checking with travis-ci.

* include 'helix/pico/back' to CI
  helix pico and helix rev2 are still available. I would like to check both by travis-ci, but here I will add a check only for helix pico.