Check binary compatibility with 7.0.1, .2, & .3 also.
Reenable travis test.
Place MiMa basis in version.sbt; make it 7.0.3; only check that in Travis.
new release step: write MiMa base version

Based on a suggestion by @gseitz
Merge pull request #500 into series/7.0.x

Test binary compatibility with releases after 7.0.0