Fix Maven dependencies

[duschu]

Added Maven Central repository and updated jsonld-java dependency in order to fix issue #18

[duschu]

The issue #18 is related to a conflict between the Maven Central and the Jitpack repositories. The issue-related project with the groupId "com.github.jsonld-java" exist in both repositories. In fact, we want it to be fetched from the official Maven Central repository. But the groupId also follows the Jitpack naming convention, thus Jitpack tries to fetch the project from github using their own dependency resolution logic. Here, a conflict occurs and the Maven build fails.

The problem can be solved by explicitly specifying Maven Central as the first repository to use for dependency resolution in the Maven POM file. In addition, I changed the dependency groupId of the jsonld-java project according to their own documentation under https://github.com/jsonld-java/jsonld-java.

In order to build the project successfully, it might be necessary to clean the local Maven repository cache on your machine, at least the /com/github/jsonld-java directory.

[bjdmeest]

This looks great, thanks! It'll be part of our next release, somewhere later this month :)