Run tox in parallel mode on Travis CI

[sbethur]

• [x] This contribution adheres to CONTRIBUTING.md.

~- [ ] I've updated CHANGELOG.md if applicable.~

~- [ ] I've added tests applicable for this pull request~

What does this Pull Request accomplish?

Run tox in parallel mode on Travis CI.

List issues fixed by this Pull Request below, if any.

None

What testing has been done?

Ran in parallel mode locally.

[marcoskirsch]

Please update PR description once you have comparatibe timing numbers.