Reduce test matrix
Reduce test matrix

As it is only required here to test using a single compiler version and
that a single version of Python is sufficient, this has been changed
accordingly.

Having said that, an additional OS has been added, macOS to test the
following combination:

    (macos, gcc) and (ubuntu, gcc)

REF:

    https://github.com/devitocodes/devito/pull/1069#discussion_r364799357

	modified:   tutorials.yml

opesci/devito,opesci/devito