Add update.sh script to easily update all the versions we support

Output looks like:
```console
$ ./update.sh
+ sed -ri 's/^(ENV NODE_VERSION ).*/\10.10.30/' 0.10/Dockerfile
+ sed -ri 's/^(ENV NODE_VERSION ).*/\10.11.13/' 0.11/Dockerfile
+ sed -ri 's/^(ENV NODE_VERSION ).*/\10.8.28/' 0.8/Dockerfile
```