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

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, 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.
Merge pull request #16 from kyanny/newrelic_rpm_3.8.1.221

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