Keep selenium==2.52.0 in Travis.

Version >=2.53.0 is breaking Unselect From List By Value.

Author: HelioGuilherme66

.travis_python_dep.sh

@@ -3,21 +3,21 @@ set -ev
 PYTHON_VERSION=`python -c "import sys; print(sys.version[:3])"`
 if [ "${PYTHON_VERSION}" = "2.6" ]; then
     pip install robotframework
-    pip install selenium
+    pip install selenium==2.52.0
     pip install decorator
     pip install docutils
     pip install future
     pip install mockito
 elif [ "${PYTHON_VERSION}" = "2.7" ]; then
     pip install robotframework
-    pip install selenium
+    pip install selenium==2.52.0
     pip install decorator
     pip install docutils
     pip install future
     pip install mockito
 elif [ "${PYTHON_VERSION}" = "3.4" ]; then
     pip install robotframework
-    pip install selenium
+    pip install selenium==2.52.0
     pip install decorator
     pip install docutils
     pip install future