Fix bug where manifest is missing its last line.

Despite the [documentation on java.util.jar.Manifest](http://docs.oracle.com/javase/6/docs/technotes/guides/jar/jar.html), the Manifest class
seems to require an endline in its input. If not, it drops the last line (at least on my
system, version reported at the bottom).

This fixes bug https://github.com/technomancy/leiningen/issues/1195

$ java -version
java version "1.7.0_21"
OpenJDK Runtime Environment (IcedTea 2.3.9) (7u21-2.3.9-0ubuntu0.12.04.1)
OpenJDK 64-Bit Server VM (build 23.7-b01, mixed mode)