Add java9 in CI build

[htynkn]

Currently dubbo support java6, java7 and java8.

All unit test pass in java9, we should consider add official support for java9 and even java10.
Currently travis support java9. java10 is in progress (travis-ci/travis-ci#9368)

As discussed in mail list, the first step is to add java9 in travis ci.

[htynkn]

Additional informaiton:
[x]All unit test can pass in java9
[x]All javadoc generate pass in java9
[] cobertura-maven-plugin doesn't work with java9

Refer to build logs: https://api.travis-ci.org/v3/job/359231113/log.txt

[lovepoem]

Now cobertura-maven-plugin is used to count the unit test coverage，we should ensure it can work

[htynkn]

@lovepoem cobertura reply on com.sun:tools:jar which java9 removed (http://openjdk.java.net/jeps/220).

I don't think cobertura is in maintain any more. Latest release is at 2015 (http://mvnrepository.com/artifact/net.sourceforge.cobertura/cobertura)

[lovepoem]

I don't think cobertura is in maintain any more. Latest release is at 2015

If so, we can consider other tools:such as coverails……

[lovepoem]

Now the java9 ci faild: #1611

[htynkn]

@lovepoem hey Xin, I figure out the reason for issue 1611. Please refer to my comment for that PR.
The issue for PR 1611 is not related to java9 travis configuration and maven-javadoc-plugin is latest version. I couldn't see much we can improve for travis CI configuration.
Could you close this issue now or after 1611 merged?

[lovepoem]

https://github.com/apache/maven-javadoc-plugin has resoled the issue in snapshot version, so we should pay attention to https://issues.apache.org/jira/projects/MJAVADOC/issues