Steal the java buildpack's release script

This will make releasing new versions easier for us.

Closes gh-723

etc/publish.sh

+#!/bin/bash
+
+set -e
+
+BP_NAME=${1:-"heroku/python"}
+
+curVersion=$(heroku buildpacks:versions "$BP_NAME" | awk 'FNR == 3 { print $1 }')
+newVersion="v$((curVersion + 1))"
+
+read -p "Deploy as version: $newVersion [y/n]? " choice
+case "$choice" in
+  y|Y ) echo "";;
+  n|N ) exit 0;;
+  * ) exit 1;;
+esac
+
+originMaster=$(git rev-parse origin/master)
+echo "Tagging commit $originMaster with $newVersion... "
+git tag "$newVersion" "${originMaster:?}"
+git push origin refs/tags/$newVersion
+
+heroku buildpacks:publish "$BP_NAME" "$newVersion"
+
+if [ $(git tag | grep -q previous-version) ]; then
+    echo "Updating previous-version tag"
+    git tag -d previous-version
+    git push origin :previous-version
+    git tag previous-version latest-version
+fi
+if [ $(git tag | grep -q latest-version) ]; then
+    echo "Updating latest-version tag"
+    git tag -d latest-version
+    git push origin :latest-version
+    git tag latest-version "${originMaster:?}"
+    git push --tags
+fi
+
+echo "Done."