Adjustments to Travis config
Adjustments to Travis config

- Try to fix build failures for Python 3.7 on amd64 (see https://travis-ci.community/t/build-error-for-python-3-7-on-two-different-projects/12895)
- Remove allowed failures, that were intended to apply to the Mac builds
  but have actually been adding more Linux builds in recent runs

ccpem/mrcfile