Allow CHEF_VERSION to include requirement specification

For example '~> 10' and '~> 0.10.6' can be used, but
'10.18.2' still forces the specific version.
Use latest Chef point releases for testing
Test also against latest Chef release

Currently 10.18.2 which is not covered by previous build matrix.
Swap Travis build matrix order

Make the most current versions to have smaller index numbers.
Test Ruby 2.0.0 only against the latest Chef version

Reduce at least a bit of the work load of the poor old Travis...