[TRAVIS] no longer run Trusty job
[TRAVIS] no longer run Trusty job

The trusty fails since Feb 2020 due to Python3 being 3.4.3, even though we ask for more recent.
See https://travis-ci.org/CCPPETMR/SIRF-SuperBuild/jobs/654024429

Trusty is too old to bother fixing this though.

Closes #329

CCPPETMR/SIRF-SuperBuild,CCPPETMR/SIRF-SuperBuild