Ensure build with java 11 and 12 works

[davidmoten]

When building with Java 11 or 12 (openjdk) the build failed with javadoc errors. This PR

• fixes the javadoc errors (stuff like <, >, & need to be escaped properly in javadoc, and mismatched parameters)
• upgrades maven-javadoc-plugin to 3.1.0 (latest)
• adds openjdk11 to the Travis build matrix

OpenJDK 12 is not available yet on Travis but I confirm the build passes with it also.

[coveralls]

Pull Request Test Coverage Report for Build 122

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.05%) to 58.855%

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Totals
Change from base Build 84:	0.05%
Covered Lines:	4556
Relevant Lines:	7741

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💛 - Coveralls

[sylvestre]

Thanks!

[sylvestre]

could you add that to the release notes? https://github.com/opencollab/jlatexmath/blob/master/CHANGELOG

[davidmoten]

could you add that to the release notes?

Done (committed directly).