Exclude OpenBLAS on osx from the build matrix

[jonlawlor]

It takes more than 10m to make openblas, and the travis builds are automatically terminated if they take too long without returning any input. Maybe the homebrew installation script does too much, maybe it should be verbose, or maybe the travis servers are slow right now, but in any case I don't want to show the build as erroring when it just timed out in a semi-redundant environment.

[jonlawlor]

PTAL @btracey @vladimir-ch

Inconsistent build results = bad

[jonlawlor]

ping @btracey @vladimir-ch

[btracey]

lgtm