Add GitHub Actions build

[beatngu13]

While getting familiar with the project for #250, I thought I'd start with contributing a CI build (couldn't finde one?) via GitHub Actions.

There are two issues, which I also experience locally:

• The POM says Java version 6, although a dependency (org.codehaus.plexus.classworlds.launcher.Launcher) requires 7. (See this build.)

• The integration tests are failing and I couldn't find a contributing guide or similar to see what is necessary to make them pass. (See this build.)

What's your take on these?

[aleksandr-m]

The POM says Java version 6, although a dependency (org.codehaus.plexus.classworlds.launcher.Launcher) requires 7.

So it is time to update pom.xml

The integration tests are failing and I couldn't find a contributing guide or similar to see what is necessary to make them pass.

They are failing with fatal: bad revision 'HEAD' error in build.log. Seems like there is no refs folder in .git. Maybe git-dir doesn't work as expected.

See https://github.com/aleksandr-m/gitflow-maven-plugin/actions/runs/221024887.

[beatngu13]

OK, gonna bump the Java version to 7 (also in the POM and elsewhere if applicable).

Regarding the integration tests: Will dig a little deeper during the weekend and hopefully come up with a fix.

[beatngu13]

Added some improvements and made the integration tests pass, unfortunately only locally. The build still fails with fatal: bad revision 'HEAD'. I have to further investigate this.

[aleksandr-m]

@beatngu13 I've found out why integrations tests were failing - it was missing git user config.

Thank you very much for initial ci script and discovering that java 7 is required!