Increase the java heap to 512M when running proguard

All of us building on Macs are running out of java heapspace during the
proguard phase of the build. This CL fixes the problem.

Change-Id: I461bb14b2ff2491e43ef8e2312e76a2a8fc0ee24
diff --git a/bin/proguard.sh b/bin/proguard.sh
index 51a7b01..31401be 100755
--- a/bin/proguard.sh
+++ b/bin/proguard.sh
@@ -9,4 +9,4 @@
 
 PROGUARD_HOME=`dirname "$0"`/..
 
-java -jar "$PROGUARD_HOME/lib/proguard.jar" "$@"
+java -Xmx512M -jar "$PROGUARD_HOME/lib/proguard.jar" "$@"