build(deps): downgrade SonarQube plugin version to 6.3.1 for compatibility

[luisgomez29]

The Sonar plugin version was adjusted to avoid the following error during analysis:

* Exception is:
org.gradle.api.tasks.TaskExecutionException: Execution failed for task ':sonarResolver'.
        at org.gradle.api.internal.tasks.properties.DefaultTaskProperties.resolve(DefaultTaskProperties.java:85)
       …
Caused by: java.io.UncheckedIOException: java.io.IOException: No such file or directory
        at org.gradle.internal.UncheckedException.throwAsUncheckedException(UncheckedException.java:63)
        ... 25 more
Caused by: java.io.IOException: No such file or directory
        at org.sonarqube.gradle.SonarResolverTask.getOutputFile(SonarResolverTask.java:97)
        at org.sonarqube.gradle.BuildFeaturesEnabledResolverTask_Decorated.getOutputFile(Unknown Source)
        at org.gradle.internal.reflect.annotations.impl.DefaultPropertyAnnotationMetadata.getPropertyValue(DefaultPropertyAnnotationMetadata.java:53)
        ... 45 more