fix javadoc unknown character errors causing javadoc gradle task to fail

[travisfw]

gradle assemble task depends on javadoc which fails due to unknown characters. The fix is to set the encoding with a parameter to the javadoc tool. Solution via http://stackoverflow.com/questions/25912190/how-to-set-an-encoding-for-the-javadoc-in-gradle

[miho]

Thanks for the fix!