[fix] fix selenium tests

.travis.yml

   - "export DISPLAY=:99.0"
   - "sh -e /etc/init.d/xvfb start"
   - npm install less less-plugin-clean-css grunt-cli
+  - export PATH=`pwd`/node_modules/.bin:$PATH
+  - ls -l $VIRTUAL_ENV
+  - ls -l $VIRTUAL_ENV/bin
   - ( cd searx/static/themes/oscar;npm install; cd - )
   - ( cd searx/static/themes/simple;npm install; cd - )
   - mkdir -p ~/drivers; export PATH=~/drivers:$PATH;
-  - GECKODRIVER_URL="https://github.com/mozilla/geckodriver/releases/download/v0.14.0/geckodriver-v0.14.0-linux64.tar.gz";
+  - GECKODRIVER_URL="https://github.com/mozilla/geckodriver/releases/download/v0.18.0/geckodriver-v0.18.0-linux64.tar.gz";
   - FILE=`mktemp`; wget "$GECKODRIVER_URL" -qO $FILE && tar xz -C ~/drivers -f $FILE geckodriver; rm $FILE; chmod 777 ~/drivers/geckodriver;
 install:
   - ./manage.sh update_dev_packages

manage.sh

     if [ -z $NOTFOUND ]; then
 	return
     fi
-    GECKODRIVER_VERSION="v0.14.0"
+    GECKODRIVER_VERSION="v0.18.0"
     PLATFORM=`python -c "import platform; print platform.system().lower(), platform.architecture()[0]"`
     case $PLATFORM in
 	"linux 32bit" | "linux2 32bit") ARCH="linux32";;

requirements-dev.txt

 transifex-client==0.12.2
 unittest2==1.1.0
 zope.testrunner==4.5.1
-selenium==3.0.1
+selenium==3.5.0