Add support for Java 11 (ETA end of September 2018)

[TheSnoozer]

• add java 11 to test matrix of travis
• update dependencies if required
• create compatibility matrix with java versions and add it to the readme (Java 1.6 -- plugin version 2.1.15; Java 1.7 -- plugin version 2.2.15)

[TheSnoozer]

Added tests for java 11 since everything seems to work fine with the current plugin version.