surefire: Use forkCount instead of deprecated forkMode.

Additionally use a property `surefire.forkCount` for this, so creation
of coverage reports may be triggered by running:

```
mvn clean test jacoco:report -Dsurefire.forkCount=1
```
Merge pull request #5 from mfriedenhagen/forkCount

surefire: Use forkCount instead of deprecated forkMode.