Warning to use java.time-based API in jib-core Retry.java

(logs from https://travis-ci.org/github/GoogleContainerTools/jib/jobs/676290173)

```
> Task :jib-core:compileJava
/home/travis/build/GoogleContainerTools/jib/jib-core/src/main/java/com/google/cloud/tools/jib/cache/Retry.java:104: warning: [PreferJavaTimeOverload] Prefer using java.time-based APIs when available. Note that this checker does not and cannot guarantee that the overloads have equivalent semantics, but that is generally the case with overloaded methods.
this.sleepMilliseconds = unit.convert(duration, TimeUnit.MILLISECONDS);
^
(see https://errorprone.info/bugpattern/PreferJavaTimeOverload)
Did you mean 'this.sleepMilliseconds = unit.convert(Duration.ofMillis(duration));'?
1 warning
```