Allow Knox site to be built with JDK 11

[exec] [ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.1:compile (default-compile) on project markbook: Compilation failure: Compilation failure:
 [exec] [ERROR] Source option 5 is no longer supported. Use 6 or later.
 [exec] [ERROR] Target option 1.5 is no longer supported. Use 1.6 or later.
 [exec] [ERROR] -> [Help 1]
 [exec] [ERROR]
 [exec] [ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
 [exec] [ERROR] Re-run Maven using the -X switch to enable full debug logging.
 [exec] [ERROR]
 [exec] [ERROR] For more information about the errors and possible solutions, please read the following articles:
 [exec] [ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MojoFailureException
 [exec] [ERROR]
 [exec] [ERROR] After correcting the problems, you can resume the build with the command
 [exec] [ERROR] mvn <goals> -rf :markbook
 [exec] Result: 1