changed CI config to skip build on docs-only-changes

Author: thombergs

.travis.yml

@@ -1,5 +1,11 @@
 before_install:
  - chmod +x gradlew
+ - |
+   if ! git diff --name-only $TRAVIS_COMMIT_RANGE | grep -qvE '(.md)|^(LICENSE)'
+   then
+     echo "Not running CI since only docs were changed."
+     exit
+   fi
 
 language: java