fix sdist upload

.travis.yml

@@ -760,12 +760,12 @@ after_success: |
       fi
 
       if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then
-        pip install --user twine
-        pip install --user --upgrade six
-
         if [[ $SDIST == 1 ]]; then
+          pip install twine
           twine upload -u ${USER} -p ${PASS} --skip-existing ${TRAVIS_BUILD_DIR}/dist/opencv*
         else
+          pip install --user twine
+          pip install --user --upgrade six
           twine upload -u ${USER} -p ${PASS} --skip-existing ${TRAVIS_BUILD_DIR}/wheelhouse/opencv*
         fi