Upgrade to JUnit 5.1.1 (#73) (#82)
Upgrade to JUnit 5.1.1 (#73) (#82)

Among other things, 5.1.1 contains crucial improvements for extension
authors:

 - New methods to query annotations on constructor/method parameters (to
   workaround a JDK bug) to `ExtensionContext`
 - Introduction of `CloseableResource` which makes implementing
   extensions like `TempDirectory` (#39) a lot simpler

Thus, this commit upgrades to 5.1.1 as the minimum required version.
CodeFX-org/junit-io