Update travis build config

- addons.sonarcloud: deprecated key branches (not supported any more)
- add missing os (linux) to config root

.travis.yml

@@ -1,27 +1,25 @@
+os: linux
 dist: xenial
 
-#
-# required to run sonarqube/sonarcloud add-on:
+# configure sonarcloud analysis
 addons:
   sonarcloud:
     organization: "jonlabelle-github"
     token:
       secure: "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"
-    branches: master
 jdk:
   - oraclejdk8
+# sonarcloud needs access to full scm history
+# https://docs.travis-ci.com/user/sonarcloud/#accessing-full-scm-history
+git:
+  depth: false
 
 language: python
 
 python:
   - "2.7"
   - "3.4"
 
-#
-# limit clone depth to 3, to speed up build:
-git:
-  depth: 3
-
 install:
   - bash scripts/install.sh