Fix Travis-CI when using rebase.

* The $TRAVIS_COMMIT_RANGE could be not existed, because the git history
  can be changed and rebased. Here we alway run Travis-CI when `git diff`
  cannot return the modified files.
* Also do not push docs update if is not PaddlePaddle/Paddle.

.travis.yml

@@ -42,12 +42,15 @@ addons:
 before_install:
   - |
     if [ ${JOB} == "BUILD_AND_TEST" ]; then
-      if ! git diff --name-only $TRAVIS_COMMIT_RANGE | grep -qvE '(\.md$)|(\.rst$)|(\.jpg$)|(\.png$)'
+      local change_list=`git diff --name-only $TRAVIS_COMMIT_RANGE`
+      if [ $? -eq 0 ]; then  # if git diff return no zero, then rerun unit test.
+        if ! echo ${change_list} | grep -qvE '(\.md$)|(\.rst$)|(\.jpg$)|(\.png$)'
         then
           echo "Only markdown docs were updated, stopping build process."
           exit
         fi
       fi
+    fi
   - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then sudo paddle/scripts/travis/before_install.linux.sh; fi
   - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then paddle/scripts/travis/before_install.osx.sh; fi
   - pip install wheel protobuf sphinx breathe recommonmark virtualenv numpy sphinx_rtd_theme

paddle/scripts/travis/docs.sh

@@ -47,6 +47,7 @@ if [ $? -eq 0 ]; then
 fi
 set -e
 
+if [ -n $SSL_KEY ]; then  # Only push updated docs for github.com/PaddlePaddle/Paddle.
   # Commit
   git add .
   git config user.name "Travis CI"
@@ -61,3 +62,5 @@ ssh-add deploy_key
 
   # Push
   git push $SSH_REPO $TARGET_BRANCH
+
+fi