Drop expressions in repository URLs and manage version of Maven Install Plugin

[mthmulders]

Following the conversation in #84, here's a pull request that does the trivial changes:

• Replace expressions in URLs with their actual values.
• Pin versions of plugins

There's one more thing that must be done. If you would attempt to make a release, it would fail with

The maven-gpg-plugin is not supported by Maven 4. Verify if there is a compatible signing solution, add -Dmaven.experimental.buildconsumer=false or use Maven 3.

The Sign Maven Plugin looks like a candidate replacement. Its website says it works on Maven 3.6 and is ready for Maven 4 with Consumer POM.

Since the Sign Maven Plugin does not look like a drop-in replacement to me, and since it is not part of the ASF Maven project, I chose not to include it (yet) in this PR. If the Eclipse EE4J project decides to adopt that plugin, it could be part of this PR.