Fixed constant merge conflicts in build.properties

Due to `build.number` and to some extent `mod_version` being situated in `build.properties`, whenever `build.properties` changes on both pahimar's and the local end, a merge conflict results.  This fixes that by making Git always keep the local version whenever a merge conflict would have occurred.  This does make for some manual updating of the file, but I think it's better than having to discard your changes in the file over and over again.
(http://git-scm.com/book/ch7-2.html#Merge-Strategies)