add automatic module name to pf4j.jar

[pinhead84]

According to this recommendation the pf4j.jar file should provide an Automatic-Module-Name in its MANIFEST.MF. This pull requests extends the pom.xml in order to add the line

Automatic-Module-Name: org.pf4j

to the MANIFEST.MF of pf4j.jar (see this commit). In case PF4J is migrated to Jigsaw (Java 9+) in the future, you can use org.pf4j in the module-info.java without getting conflicts with application, that already make use of Jigsaw (Java 9+).

I didn't make any changes to the demo application. Because in most cases the demo libraries aren't reused by other applications.

[coveralls]

Pull Request Test Coverage Report for Build 455

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.1%) to 52.613%

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Totals
Change from base Build 454:	-0.1%
Covered Lines:	906
Relevant Lines:	1722

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 455

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.1%) to 52.613%

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Totals
Change from base Build 454:	-0.1%
Covered Lines:	906
Relevant Lines:	1722

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💛 - Coveralls