Add SonarQube back to Travis builds

[peter-tackage]

Due to plugin incompatibilities, I've disabled the SonarQube check in the Travis builds as a part of https://github.com/futurice/freesound-android/pull/131

Once the issues are resolved, we should re-instate it. The following links describe and track the issue:

• https://groups.google.com/forum/#!topic/sonarqube/WN_zMNKfkAU
• https://issuetracker.google.com/issues/62688864
• https://jira.sonarsource.com/browse/SONARGRADL-41

[peter-tackage]

This PR to SonarSource addresses the issue: https://github.com/SonarSource/sonar-scanner-gradle/pull/33

Once it's merged and a new version of the plugin is release, then we can proceed.

[peter-tackage]

Resolved via #161