Travis: fix the latest breakage - use group:

0xc0170 · 0xc0170 · commit 42a6dd00d81f · 2017-09-08T13:50:56.000+01:00
See https://blog.travis-ci.com/2017-08-29-trusty-image-updates
diff --git a/.travis.yml b/.travis.yml
@@ -1,6 +1,6 @@
 python:
   - "2.7"
-
+group: deprecated-2017Q3
 script:
   - mkdir BUILD
 # Assert that the Doxygen build produced no warnings.
