Fix "uninitialized constant NewRelic::MetricSpec (NameError)" for newrelic_rpm 3.8.1.221

Minimum replicate code: https://gist.github.com/kyanny/ea06c6eb76500c3ed15a

At newrelic_rpm 3.8.0.218, dependency is resolved like this:

    - newrelic-grape/instrument
      - new_relic/agent/instrumentation/controller_instrumentation
        - new_relic/agent/transaction
          - new_relic/agent/transaction/pop
            - new_relic/agent/instrumentation
              - new_relic/agent
                - new_relic/metric_spec

But at newrelic_rpm 3.8.1.221, new_relic/agent/transaction/pop was deleted ([e709be6](https://github.com/newrelic/rpm/commit/e709be61edd2abb63f524c49dab13abc346005fd)), so dependency was broken.
Therefore newrelic-grape no longer loads `NewRelic::MetricSpec` and raises exception at application launch timing.

To solve this dependency problem, require new_relic/agent/instrumentation clearly.