chore: set Node.js compatibility to v6+

- drop support for versions less than Node v6 via engines
- update integration tests to also test newer versions

integration-testing/multi-nodejs-test/test.sh

@@ -17,7 +17,7 @@ cd "$( dirname "$( readlink -f "$0" )" )" || exit 1
 unset npm_config_prefix
 
 echo "Handlebars should be able to run in various versions of NodeJS"
-for i in  0.10 0.12 4 5 6 7 8 9 10 11 ; do
+for i in 6 7 8 9 10 11 12 13; do
     rm target node_modules package-lock.json -rf
     mkdir target
     nvm install "$i"