feat(builder): support submodules in buildpacks

some buildpacks require external git submodules to pulled in.
for example: https://github.com/cloudfoundry/ruby-buildpack

Author: lanej

builder/rootfs/usr/local/src/slugbuilder/builder/build.sh

@@ -111,6 +111,8 @@ if [[ -n "$BUILDPACK_URL" ]]; then
         git clone --quiet "$url" "$buildpack"
         pushd "$buildpack" &>/dev/null
             git checkout --quiet "$committish"
+            git submodule init --quiet
+            git submodule update --quiet --recursive
         popd &>/dev/null
     fi