Cache Gradle and not really Maven

Artifacts that Gradle caches from Maven Central go in
~/.gradle/caches/modules-2, not ~/.m2. We only need .m2 for the
locally-installed Netty.
diff --git a/buildscripts/make_dependencies.sh b/buildscripts/make_dependencies.sh
index 4af54b2..afafaa8 100755
--- a/buildscripts/make_dependencies.sh
+++ b/buildscripts/make_dependencies.sh
@@ -21,7 +21,7 @@
 # Make and install netty
 pushd lib/netty
 BUILD_NETTY=1
-NETTY_REV_FILE="$HOME/.m2/netty-ver"
+NETTY_REV_FILE="$HOME/.m2/repository/io/netty/netty-ver"
 REV="$(git rev-parse HEAD)"
 if [ -f "$NETTY_REV_FILE" ]; then
   REV_LAST="$(cat "$NETTY_REV_FILE")"