Ask Travis to install 3.5 explicitly

.travis.yml

@@ -10,7 +10,7 @@ git:
 
 before_install:
 - sudo apt-get update
-- sudo apt-get install python3
+- sudo apt-get install python3.5
 
 # test-and-build does the installation, so tell travis to skip the
 # installation step

scripts/test-and-build.sh

@@ -4,7 +4,7 @@ set -ex
 
 cd `dirname $0`/..
 
-virtualenv -p python3 env
+virtualenv -p python3.5 env
 . env/bin/activate
 
 # Print out the python versions for debugging purposes