Switch from net.imagej:ij to gov.nih.imagej:imagej

This prevents one more indirect dependency on maven.imagej.net.  With this branch, I can now successfully do this locally:

```
rm -rf ~/.m2/repository/*
mvn clean
mvn
```
after adding ```127.0.0.1 maven.imagej.net``` to my ```/etc/hosts``` file; I'd expect Travis and Jenkins to be happy as well.