title: NoClassDefFoundError when upgrading Apptentive from version 1.7.3 version to 2.1.1 version
body: <p>I am trying to upgrade Apptentive from 1.7.3 version to latest 2.1.1 version.
  However whenever I call method<br />
  Apptentive.showMessageCenter(getActivity()), to launch Apptentive message dialog, it crashes and gives NoClassDefFoundError for ViewActivity.
  Logs are as below:</p>
  <blockquote>
  <p>java.lang.NoClassDefFoundError: com.apptentive.android.sdk.ViewActivity</p>
  <p>01-08 11:30:41.837 E/AndroidRuntime(30049):   at com.apptentive.android.sdk.module.engagement.EngagementModule.launchInteraction(EngagementModule.java:75)</p>
  <p>01-08 11:30:41.837 E/AndroidRuntime(30049):   at com.apptentive.android.sdk.module.engagement.EngagementModule.doEngage(EngagementModule.java:64)</p>
  <p>01-08 11:30:41.837 E/AndroidRuntime(30049):   at com.apptentive.android.sdk.module.engagement.EngagementModule.engage(EngagementModule.java:53)</p>
  <p>01-08 11:30:41.837 E/AndroidRuntime(30049):   at com.apptentive.android.sdk.module.engagement.EngagementModule.engageInternal(EngagementModule.java:31)</p>
  <p>01-08 11:30:41.837 E/AndroidRuntime(30049):   at com.apptentive.android.sdk.ApptentiveInternal.showMessageCenterInternal(ApptentiveInternal.java:191)</p>
  <p>01-08 11:30:41.837 E/AndroidRuntime(30049):   at com.apptentive.android.sdk.Apptentive.showMessageCenter(Apptentive.java:635)</p>
  <p>01-08 11:30:41.837 E/AndroidRuntime(30049):   at com.apptentive.android.sdk.Apptentive.showMessageCenter(Apptentive.java:619)</p>
  </blockquote>
  <p>My code in build.gradle file is:</p>
  <pre><code>compile 'com.apptentive:apptentive-android:2.1.1@aar'
  </code></pre>
  <p>and java code is:</p>
  <pre><code>    Button writeUs = (Button) fitnessSyncDialog.findViewById(R.id.button_click_write_us);
  writeUs.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
          Apptentive.showMessageCenter(getActivity());
      }
  });
  </code></pre>
  <p>Same code is working with 1.7.3 version.
  has anyone faced this problem or Can anyone suggest what problem can be here?</p>
  <p>Thanks in advance for any valuable suggestions.</p>