bpo-32802: Fix Travis build (GH-5589)

Fix bug in travis configuration where it did not run the tests when
a change includes both code and doc changes.
diff --git a/.travis.yml b/.travis.yml
index d7387e5..98d6b9a 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -97,7 +97,7 @@
       echo "Files changed: "
       echo $files_changed
 
-      if ! echo $files_changed | grep -qvE '(\.rst$)|(^Doc)|(^Misc)'
+      if ! echo "$files_changed" | grep -qvE '(\.rst$)|(^Doc)|(^Misc)'
       then
         echo "Only docs were updated, stopping build process."
         exit