gitignore `.ruby-version` & `bin/`

The bin directory may contain bundler binstubs for development
Test against Faraday 0.7.x on CI
Enable coverage & quality test only with default Gemfile

Speeds up CI build for non-standard Gemfiles
Test with Ruby 2.1.1 on CI
Show all failing tests
Improve mashify/rashify specs

The specs used to be isolated from the `call(env)` cycle by means of
using the `on_complete` method directly. Since that method is not
considered part of the public API of Faraday::Response::Middleware, this
switches away from using that method and makes the middleware process
responses the standard way, via `call()`.

This gets rid of the need for "integration tests" because the middleware
is exercised exactly as it would during a real request/response cycle.
Tolerate 98.9% code coverage

Coverage dropped slightly below 99% because in caching middleware there
is a code path that is only effective during parallelism, but it's not
trivial to setup parallel requests to exercise that scenario.
Enable RackCompatible specs on Rubinius

It used to be that those failed because of a Rubinius bug
https://github.com/rubinius/rubinius/issues/1522

But it's been 2 years since then, so assume that it's fixed.