[CI] Travis: Upgrade to Xenial and add pygit2

.travis.yml

@@ -1,5 +1,6 @@
 sudo: false
 language: python
+dist: xenial
 python:
   - "2.7"
   - "3.3"
@@ -8,7 +9,7 @@ python:
   - "3.6"
 before_install:
   - sudo apt-get -qq update
-  - sudo apt-get install -y task libgit2-dev
+  - sudo apt-get install -y task pygit2
 install:
   - pip install i3ipc
   - pip install psutil
@@ -16,7 +17,6 @@ install:
   - pip install -U coverage==4.3
   - pip install codeclimate-test-reporter
   - pip install taskw
-  - pip install pygit2
 script:
   - nosetests -v --with-coverage --cover-erase tests/
   - CODECLIMATE_REPO_TOKEN=40cb00907f7a10e04868e856570bb997ab9c42fd3b63d980f2b2269433195fdf codeclimate-test-reporter