fix the build for PRs (was always using master)

See comments on this travis-ci ticket
travis-ci/travis-ci#6652

.travis.yml

@@ -11,9 +11,9 @@ before_install:
 script:
  - docker run  --interactive --tty bfarnham/quasar:quasar-open62541 /bin/sh -c "
      echo '********************************************************************' ;
-     echo branch $TRAVIS_BRANCH ;
+     echo branch ${TRAVIS_PULL_REQUEST_BRANCH:-$TRAVIS_BRANCH} ;
      echo '********************************************************************' ;
-     git clone -b $TRAVIS_BRANCH https://github.com/quasar-team/CanModule.git ;
+     git clone -b ${TRAVIS_PULL_REQUEST_BRANCH:-$TRAVIS_BRANCH} https://github.com/quasar-team/CanModule.git ;
      mkdir CanModule/build-release && cd CanModule/build-release ;
      apt-get install --quiet --assume-yes libsocketcan ;
      apt-get install --quiet --assume-yes libsocketcan-dev ;