[TRAVIS] change setting of PY_EXE on OSX
[TRAVIS] change setting of PY_EXE on OSX

Use the simpler mechanism introduced in SIRF earlier and which seems to work fine there.

Take the opportunity to sync parts of the comments with the SIRF .travis.yml

CCPPETMR/SIRF-SuperBuild,CCPPETMR/SIRF-SuperBuild