Simplify Travis CI build matrix config

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Add spacing to .travis.yml config for readability

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Reorder .travis.yml config according to lifecycle

[ci skip]

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Merge pull request #2206 from opf/feature/travisci-simplify-matrix