Parallelize Travis suite

Runs dist task, unit/integration tests, and Selenium tests as three
separate suites. We were often running into timeouts before because
of Travis-CI's hard 50-minute cap.

Also make setting some of the environment variables a bit cleaner:
INTEGRATION_LOG_FILE, SCREENSHOT_ON_ERROR, and JAVA_OPTS (used to
pass the MySQL JDBC connection string).
Merge pull request #14 from anarchivist/parallelize-travis-once-and-for-all

Parallelize Travis suite