Update errorprone config for Java 10

[AustinShalit]

Basically followed the instructions here: google/error-prone#860

@auscompgeek if you wouldn't mind checking this.

[auscompgeek]

Well, it builds on Java 10. Can't say the same for the CI running Java 8 though.

[AustinShalit]

Yea... I think I just fixed that. If you wouldn't mind trying again.

[auscompgeek]

Works on Java 10, but CI still seems unhappy. :(

[AustinShalit]

Oops. I missed a critical piece of docs: "Error Prone requires at least a JDK 9 compiler to be used as a javac plugin." So this should be merged when we decide to go to Java 10.

[auscompgeek]

I guess we can deal with the warnings until that happens then.

[ThadHouse]

Is there no way to support both Java 8 and Java 10? We kind of want both to be supported by the build setup.

[AustinShalit]

The trusty if statement worked well here

[auscompgeek]

Nice.

[tbroyer]

Note: net.ltgt.errorprone-javacplugin version 0.5 now works out of the box with JDK 8 (you just have to configure an errorproneJavac dependency to com.google.errorprone:javac) so you no longer need to conditionally switch between two plugins (and two ways of configuring Error Prone).
This will become the next version of net.ltgt.errorprone (so be prepared to switch again in a few weeks/months, back to net.ltgt.errorprone; only the plugin ID will change)

[AustinShalit]

Awesome! We'll keep an eye out for that.