Supress zlib valgrind error.

zlib 1.2.3 uses uninitialized memory in a safe way for performance, which valgrind detects as an error. This is fixed in zlib 1.2.4, but the Travis build servers uses an old version.

http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=577135
http://stackoverflow.com/questions/12483612/libzip-example-contains-uninitialised-values-when-checked-with-valgrind