travis: switch to xenial

QubesOS/qubes-issues#4613

.travis.yml

@@ -1,5 +1,5 @@
 sudo: required
-dist: trusty
+dist: xenial
 language: python
 python:
   - '3.5'
@@ -11,3 +11,9 @@ script:
  - ~/qubes-builder/scripts/travis-build
 env:
  - DIST_DOM0=fc25 USE_QUBES_REPO_VERSION=4.0 USE_QUBES_REPO_TESTING=1
+
+# don't build tags which are meant for code signing only
+branches:
+  except:
+    - /.*_.*/
+    - build