Use scijava-common 2.7.1-SNAPSHOT specifically.

So that the discovery mecanism works out of the box, even for small
test snippets.
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
Fix DetectorProvider fields.

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
Simplify the DetectorProvider.

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>