travis: flag codecov.io uploads with 'unittests' flag

We may later upload also integrations tests coverage, ease
distinguishing those two.

.travis.yml

@@ -14,7 +14,7 @@ env:
  - DIST_DOM0=fc25 USE_QUBES_REPO_VERSION=4.0 USE_QUBES_REPO_TESTING=1
 
 after_success:
-  - codecov
+  - codecov -F unittests
   - ~/qubes-builder/scripts/travis-deploy
 
 # don't build tags which are meant for code signing only