Update ee version in travis to pull test images

Author: mrrobot47

ci/prepare.sh

@@ -10,6 +10,7 @@ rm ee
 cd ..
 git clone https://github.com/EasyEngine/easyengine.git easyengine --depth=1
 cd easyengine
+echo 'travis_test' > VERSION
 
 # Copy tests to EE repo
 rm -r features