Clean up travis configuration
Clean up travis configuration

Remove unnecessary custom settings.xml file and cache cleanup:
- Committers can clean caches as needed from the Travis CI UI
- Maven 3.5.2 (current) uses https by default for Maven Central

keith-turner/accumulo,phrocker/accumulo-1,mjwall/accumulo,ivakegg/accumulo,ctubbsii/accumulo,ivakegg/accumulo,lstav/accumulo,ctubbsii/accumulo,keith-turner/accumulo,milleruntime/accumulo,mjwall/accumulo,keith-turner/accumulo,lstav/accumulo,keith-turner/accumulo,apache/accumulo,mjwall/accumulo,mjwall/accumulo,phrocker/accumulo-1,keith-turner/accumulo,keith-turner/accumulo,milleruntime/accumulo,mjwall/accumulo,lstav/accumulo,apache/accumulo,apache/accumulo,phrocker/accumulo-1,phrocker/accumulo-1,apache/accumulo,ivakegg/accumulo,lstav/accumulo,apache/accumulo,apache/accumulo,mjwall/accumulo,ctubbsii/accumulo,ivakegg/accumulo,milleruntime/accumulo,ctubbsii/accumulo,ctubbsii/accumulo,ctubbsii/accumulo,lstav/accumulo,milleruntime/accumulo,milleruntime/accumulo,mjwall/accumulo,lstav/accumulo,keith-turner/accumulo,lstav/accumulo,ivakegg/accumulo,milleruntime/accumulo,ivakegg/accumulo,phrocker/accumulo-1,ivakegg/accumulo,phrocker/accumulo-1,apache/accumulo,ctubbsii/accumulo,phrocker/accumulo-1,milleruntime/accumulo