Finish upgrading Gradle wrapper from 2.0 => 2.1

Commit 4d4787d updated the Gradle wrapper config from 2.0 => 2.1, but
did not actually update the wrapper properties file (forgot to run
`gradle wrapper`). This commit finishes what was started there.
Work around outdated OracleJDK8 on Travis CI server

The Gradle JavaFX plugin requires Oracle JDK 8u20 or better. Travis CI
currently runs something older, and does not have a roadmap for
upgrading. This commit experiments with what Travis support suggested as
a workaround.

See #100