Allow bundler up to 2.0

As part of making One Eye easier to install, we should allow reasonable versions of Bundler. 

This change would allow versions of Bundler up to `2.0.0` (currently on `1.7.2`).

These builds succeed; in fact, in `apache/thrift`, they've removed the version entirely.

https://github.com/apache/thrift/blob/master/lib/rb/thrift.gemspec#L34

I think we're probably safer allowing up to the next breaking change, though.