Split housekeeping scripts into separate Travis job (#1688)

.travis.yml

@@ -1,30 +1,27 @@
 language: python
+dist: xenial
+  
+python:
+  - 2.7
+  - 3.5
+  - 3.6
+  - 3.7
+  - nightly
 
 matrix:
   include:
-    - python: 2.7
-      dist: trusty
+    # Python 3.4 not supported in xenial
     - python: 3.4
       dist: trusty
-    - python: 3.5
-      dist: trusty
-    - python: 3.6
-      dist: trusty
-    - python: 3.7
-      dist: xenial
-    - python: nightly
-      dist: trusty
+    - env: JOB=HOUSEKEEPING
+      install: ./bin/fetch-configlet
+      before_script: ./bin/check-readmes.sh
+      script: ./bin/configlet lint .
   allow_failures:
     - python: nightly
 
-install:
-  - pip install -r requirements-travis.txt
-
 before_script:
   - flake8
-  - ./bin/fetch-configlet
-  - ./bin/configlet lint .
-  - ./bin/check-readmes.sh
   
 script: 
   - ./test/check-exercises.py