Cache the select2 library in CI

[dafeder]

We get failures sometimes when we run the ddev select2 command in CI. I'm actually not sure why but could be a rate limit from github or something. CircleCI can cache dependencies based on a key, so we can take advantage of that.

This also modifies the select2 ddev command to exit if the library is already there.

The cache key in circleCI is a hash of the command itself, since the command targets a specific commit, if we ever update that commit ID or anything else about the logic in the script it will invalidate the cache.