Avoid version ranges. They are evil!

See #89. I was unable to build a WAR project that used to build fine with this plugin due to an error about a SNAPSHOT parent POM. And when I checked out _this_ plugin project from GitHub, I could not build it either, due to the same error. (Tried Maven 2.2.1, 3.0.5, 3.1.0.) Apparently resolved with this patch.