Pull before push to avoid race conditions to other builds

tools/travis/travis_push_release.sh

@@ -51,7 +51,7 @@ git config --global user.email "open62541-travis-ci@users.noreply.github.com"
 git config --global user.name "Open62541 travis-ci"
 git config --global push.default simple
 git commit -am "added release files and updated releases webpage by travis-ci [ci skip]"
-git push https://$GITAUTH@github.com/open62541/open62541-www
+git pull && git push https://$GITAUTH@github.com/open62541/open62541-www
 
 cd ..
 rm -rf open62541-www