OMEROFormat: tweak javadoc to mention read + write
ij-omero-server: use latest version of pom-scijava

This updates the following versions:
  * ImageJ2: 2.0.0-beta-7.2 to 2.0.0-beta-7.3
  * ImgLib2: 2.0.0-beta-11 to 2.0.0-beta-15
  * SCIFIO: 0.4.1 to 0.6.0

In particular, it updates the net.imglib2.meta API to the latest
version, which includes better support for axes. We can now store the
physical calibration of each axis with the axis itself.