[travis] remove pygit2

.travis.yml

@@ -14,7 +14,7 @@ install:
   - pip install i3-py Pillow Babel DateTime python-dateutil
   - pip install docker feedparser i3ipc
   - pip install netifaces power
-  - pip install psutil pygit2 pytz
+  - pip install psutil pytz
   - pip install requests simplejson
   - pip install socket speedtest suntime
   - pip install tempfile tkinter tzlocal