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

Fix bug in travis configuration where it did not run the tests when
a change includes both code and doc changes.
(cherry picked from commit 32921f9)

Co-authored-by: Stéphane Wirtel <stephane@wirtel.be>

.travis.yml

@@ -97,7 +97,7 @@ before_script:
       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