[travis] Separate builds for core and download

.travis.yml

@@ -8,7 +8,12 @@ python:
   - "3.5"
   - "3.6"
 sudo: false
-script: nosetests test --verbose
+env:
+  - YTDL_TEST_SET=core
+  - YTDL_TEST_SET=download
+before_script:
+  - chmod +x ./devscripts/run_tests.sh
+script: ./devscripts/run_tests.sh
 notifications:
   email:
     - filippo.valsorda@gmail.com