Add PR friendly travis ci config

Author: patrickfav

.travis.yml

@@ -17,7 +17,8 @@ before_install:
   - tar xvf secrets.tar
 
 script:
-  - ./mvnw clean install -X -Djarsigner.skip=false checkstyle:check
+  - 'if [ "$TRAVIS_PULL_REQUEST" != "false" ]; then ./mvnw clean install -X -Djarsigner.skip=true checkstyle:check; fi'
+  - 'if [ "$TRAVIS_PULL_REQUEST" = "false" ]; then ./mvnw clean install -X -Djarsigner.skip=false checkstyle:check; fi'
 
 after_success:
   - ./mvnw test jacoco:report coveralls:report