Not only keep public classes, but all of them

This fixes sqlcipher#164 when using the ProGuard version that ships with the
Android Gradle plugin.

Author: sschuberth

proguard-project.txt

@@ -53,6 +53,6 @@
     public static <fields>;
 }
 
--keep public class net.sqlcipher.** {
+-keep class net.sqlcipher.** {
     *;
 }