Upgrade Voice SDK Android dependency.
Upgrade Voice SDK Android dependency.

This avoids a build error:

```
> Task :app:mergeDebugResources FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':app:mergeDebugResources'.
> Could not resolve all files for configuration ':app:debugRuntimeClasspath'.
   > Could not find com.getkeepsafe.relinker:relinker:1.3.1.
     Searched in the following locations:
       - https://dl.google.com/dl/android/maven2/com/getkeepsafe/relinker/relinker/1.3.1/relinker-1.3.1.pom
       - https://repo.maven.apache.org/maven2/com/getkeepsafe/relinker/relinker/1.3.1/relinker-1.3.1.pom
     Required by:
         project :app > com.twilio:voice-android:5.6.1

```
jefflinwood/twilio-voice-phonegap-plugin,jefflinwood/twilio-voice-phonegap-plugin