The building tools' version in README doc is outdated

1. The JDK version in doc requires >= 1.8, but actually we enforced it to 1.11 after DBZ-2870
2. The maven version in doc requires >= 3.2.1, but my version 3.2.5 is not worked with a range exception remind me version  <= 3.6.3, I upgrade maven to 3.8.1 to address this problem.

        And due to the network reason I used some maven mirrors that used http protocol, but the maven enforce checker plugin will stop this unsafe http link,  I finally work around with 
-Denforcer.skip=true, I think it's valuable if we can also add this note to README docs.