Release stripe-ruby 2.0

[brandur]

I realized as I was authoring #492 that I wanted to make a series of changes that will necessitate dropping Ruby 1.9, so I want to bump our major version to do so, and also introduce a couple other improvements to incentive the upgrade.

So far the planned work items are:

• Drop Ruby 1.9 (Remove support for Ruby 1.9 #495)
• Switch test suite over to use Webmock (Use Webmock instead of Mocha for HTTP stubbing #496)
• Introduce configurable HTTP client (and as a result, connection pooling) (Make request ID and response information available on all API calls #492, fixes Configurable HTTP client #313)
• Allow access to response objects (Make request ID and response information available on all API calls #492, fixes Making Request Id available? #484)

[morgoth]

I guess this can be closed now, as we have 2.1.0 already?

[brandur-stripe]

Oops, yep. I forgot to close out this issue. Thanks!