switched from java 11 to java 8

[blackwiz4rd]

This PR targets #357

[JakeWharton]

We can downgrade, but:

• We should not use toolchains. Toolchains are bad. We only need to set source/target appropriately.
• We probably will never release a new version of this library so I would temper your expectations if you choose to continue to contribute.

[blackwiz4rd]

Hello,
it's ok, if you don't plan to include this change you can close it.
Regards.