Remove obsolete references to Jenkins and Travis

We don't use either of these CI systems any more.

bin/install-python

@@ -15,22 +15,6 @@
 #
 # $ ./bin/install-python
 
-# Exit if we're running on Travis CI.
-# On Travis we just want to use the versions of Python provided in the Travis
-# VM, even though they may not have be the same patch versions as in
-# .python-version.
-if [ "$TRAVIS" = "true" ]
-then
-  exit
-fi
-
-# Exit if we're running on Jenkins.
-# On Jenkins we run the tests in Docker and we just want to use the versions of
-# Python provided in the Docker container.
-if [ -n "${JENKINS_URL+set}" ]; then
-  exit
-fi
-
 # Loop over every $python_version in the .python-version file.
 while IFS= read -r python_version
 do