V2 references Faraday::Error::ConnectionFailed, which doesn't exist in newer versions of Faraday

[patbl]

A long time ago, Faraday::Error::ConnectionFailed was moved to Faraday::ConnectionFailed. This mismatch results in the following error:

Check failed to run: uninitialized constant Faraday::Error::ConnectionFailed
Did you mean?  Faraday::ConnectionFailed

Here's the line that raises the exception: https://github.com/librato/librato-metrics/blob/v2.1.2/lib/librato/metrics/middleware/retry.rb#L19

The error class was updated on the 1.x branch here, but the 2.x branch still uses the old class.