Bump Excon to 0.40.0

This pulls in out-of-the-box support for HTTP proxies running on Unix sockets.

Here's the full change log since from 0.39.5 to 0.40.0; mostly minor test stuff and bug fixes:

```
0.40.0 10/06/2014
=================

fix support for specifying ssl_ca_path
more consistent response_block/response.body behavior for mocks
add support for proxies running on unix domain sockets

0.39.6 09/22/2014
=================

pretty print stub not found errors
```