Fix Travis: remove --use-mirrors for pip (#609)

* Travis: use sudo: false (container based builds)

* Travis: remove --use-mirrors for pip-install

The option is not valid anymore with pip 8.1.2 apparently
(https://travis-ci.org/davidhalter/jedi-vim/builds/152006664).

.travis.yml

@@ -1,6 +1,7 @@
+sudo: false
 language: python
 install:
-    - pip install --quiet --use-mirrors pytest
+    - pip install pytest
 script:
     - vim --version
     - py.test