cloudfront-invalidator incompatible with Ruby 2.3

Just updated to Ubuntu 16.04

```
$ ruby -v
ruby 2.3.0p0 (2015-12-25) [x86_64-linux-gnu]

$ cloudfront-invalidator
-bash: /usr/local/bin/cloudfront-invalidator: /usr/bin/ruby2.1: bad interpreter: No such file or directory
```

Seems cloudfront-invalidator needs work before it is compatible with Ruby 2.3.
