Add support for other jdks

[snuyanzin]

Based on sqlline's HOWTO article [1]

Make sure mvn clean install and mvn site pass under JDK 1.7, 8, 9, and 10.
it makes sense to have all these jdks on travis

unfortunately oracle jdk1.7 is no longer available on travis as mentioned in the issue [2] however openjdk could be used as mentioned in the same issue

[1] https://github.com/julianhyde/sqlline/blob/master/HOWTO.md
[2] travis-ci/travis-ci#7964

[julianhyde]

Fixed in 63b8630.