Fix documentation build, take 2

support/travis-build.py

@@ -2,7 +2,7 @@
 # Build the project on Travis CI.
 
 from __future__ import print_function
-import errno, os, re, shutil, sys, tempfile, urllib
+import errno, os, re, shutil, subprocess, sys, tempfile, urllib
 from subprocess import call, check_call, check_output, Popen, PIPE, STDOUT
 
 def rmtree_if_exists(dir):