[TRAVIS] OSX: find latest openblas version (#357)
[TRAVIS] OSX: find latest openblas version (#357)

`brew` can change versions on packages due to dependencies etc. In addition, `brew` gives us a symlink at `/usr/local/opt/openblas`. So, we now use that.

Fixes #356
CCPPETMR/SIRF-SuperBuild,CCPPETMR/SIRF-SuperBuild