Exclude superfluous LICENSE from jar

Exclude META-INF/LICENSE from jar, since a directory named
'license/' is also created, and this causes problems on systems
with case-insensitive filesystems. The origin of these files is
unclear; perhaps this plugin overlays the META-INF directories of
all dependencies. LICENSE and LICENSE.txt (which will remain) are
identical copies of the Apache 2.0 license, so there is no legal
issue at stake.

Closes #22
Refs #17