Update LICENSE.txt to BSD-2

7fb0f0c changed the license in the `pom.xml` and this should be reflected in `LICENSE.txt`. I c&p'ed from [scijava-common](https://github.com/scijava/scijava-common/blob/ade6894e29a4400be2fb54ce5e02a6ce9d792fd7/LICENSE.txt) and adapted the Copyright year to 2009 - 2019 (I guess that is the right thing to do).

Author: hanslovsky