build with python 3.5 rather than 3.4 (#43)

and using xenial rather than trusty

.travis.yml

@@ -1,9 +1,11 @@
 language: python
 python:
     - "2.7"
-    - "3.4"
-dist: trusty
+    - "3.5"
+dist: xenial
 
+before_install:
+    - pip install --upgrade pip
 
 install: "pip install -r dev_requirements.txt"
 
@@ -24,13 +26,13 @@ deploy:
       secure: "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"
     on:
       tags: true
-      python: "3.4"
+      python: "3.5"
   - provider: pages
     skip_cleanup: true
     github_token: $GHPAGES_TOKEN
     local_dir: docs/_build/html
     target_branch: gh-pages
     on:
       tags: true
-      python: "3.4"
+      python: "3.5"