travis: add fc30, switch to xenial

QubesOS/qubes-issues#4845
QubesOS/qubes-issues#4613

.travis.yml

@@ -1,10 +1,17 @@
 sudo: required
-dist: trusty
+dist: xenial
 language: generic
 install: git clone https://github.com/QubesOS/qubes-builder ~/qubes-builder
 script: ~/qubes-builder/scripts/travis-build
 env:
  - DISTS_VM=fc29 USE_QUBES_REPO_VERSION=4.1 USE_QUBES_REPO_TESTING=1
+ - DISTS_VM=fc30 USE_QUBES_REPO_VERSION=4.1 USE_QUBES_REPO_TESTING=1
  - DISTS_VM=stretch USE_QUBES_REPO_VERSION=4.1 USE_QUBES_REPO_TESTING=1
  - DISTS_VM=buster USE_QUBES_REPO_VERSION=4.1 USE_QUBES_REPO_TESTING=1
  - DISTS_VM=centos7 USE_QUBES_REPO_VERSION=4.1 USE_QUBES_REPO_TESTING=1
+
+# don't build tags which are meant for code signing only
+branches:
+  except:
+    - /.*_.*/
+    - build