Fix RSpec 3 compatibility (partially)

- `its` is deprecated, require rspec-its instead
- `be_false/be_true` tests for `false?/true?` now, switch to `be(false|true)`
Address deprecation of setting and special-case Rack adapter config.
Resolve deprecations in dispatcher spec.
Fix failing specs and deprecations in response_spec.
Fix errors and deprecations in adapter_spec
Resolve errors and deprecations in trace fsm spec.
Fix deprecation warnings in flow_spec
Fix deprecation warnings in trace_spec
Resolve deprecation warnings in events_spec
Resolve deprecation warnings in helpers_spec
Resolve deprecation warnings in application_spec
Resolve deprecation warnings in adapter_lint
Require rspec > 3.0 since we're using the new syntax
Resolve deprecation warnings in configuration_spec
Convert specs to RSpec 3.0.0 syntax with Transpec

This conversion is done by Transpec 2.2.4 with the following command:
    transpec

* 132 conversions
    from: obj.should
      to: expect(obj).to

* 99 conversions
    from: == expected
      to: eq(expected)

* 68 conversions
    from: it { should ... }
      to: it { is_expected.to ... }

* 14 conversions
    from: it { should_not ... }
      to: it { is_expected.not_to ... }

* 8 conversions
    from: obj.should_receive(:message)
      to: expect(obj).to receive(:message)

* 5 conversions
    from: obj.stub(:message)
      to: allow(obj).to receive(:message)

* 3 conversions
    from: obj.should_not
      to: expect(obj).not_to

* 2 conversions
    from: obj.stub(:message => value)
      to: allow(obj).to receive_messages(:message => value)

* 1 conversion
    from: =~ /pattern/
      to: match(/pattern/)

* 1 conversion
    from: failure_message_for_should { }
      to: failure_message { }

* 1 conversion
    from: failure_message_for_should_not { }
      to: failure_message_when_negated { }

* 1 conversion
    from: obj.should_not_receive(:message)
      to: expect(obj).not_to receive(:message)

For more details: https://github.com/yujinakayama/transpec#supported-conversions
Fix last deprecation that RSpec didn't catch.
Don't define module_functions for HeaderNegotiation

Makes render_error() consistent with other places which
call the ensure_* helpers.
Shrink Trace::FSM's surface

Relieve Decision::FSM from the knowledge of Trace::FSM's API,
and shrink the surface to only #tracing_enabled?.