CI: change to Ubuntu Xenial

since deb.torproject.org does not support anymore Trusty.

.travis.yml

 sudo: true
+dist: xenial
 language: python
 python:
   - "3.5"
@@ -14,8 +15,7 @@ script:
 before_install:
     - gpg --version
     - sudo apt-key add tests/deb.torproject.org.asc
-    - echo "deb https://deb.torproject.org/torproject.org trusty main" | sudo tee -a /etc/apt/sources.list
-    - echo "deb-src https://deb.torproject.org/torproject.org trusty main" | sudo tee -a /etc/apt/sources.list
+    - echo "deb https://deb.torproject.org/torproject.org xenial main" | sudo tee -a /etc/apt/sources.list
     - sudo apt-get update -qq
     - sudo apt-get install tor -y