Avoid ssh passphrase prompt

Author: Matthias Koeppe

.travis-deploy-doc.sh

@@ -10,7 +10,7 @@ if [[ "${TRAVIS_PULL_REQUEST}" == "false" && "${TRAVIS_BRANCH}" == "${DEPLOY_DOC
     fi
     chmod 600 .travis_ci_gh_pages_deploy_key
     eval `ssh-agent -s`
-    ssh-add .travis_ci_gh_pages_deploy_key
+    ssh-add .travis_ci_gh_pages_deploy_key < /dev/null
     rm -Rf gh-pages
     git clone --depth 1 git@github.com:${DEPLOY_DOC_TO_REPOSITORY}.git --depth 1 --branch=gh-pages gh-pages
     BUILT_DOCS_DIR=`cd docs/build/html && pwd`