Fix CI (#9)

* Remove duplicate line specifying language

* Try using pip3

* Revert to pip

* Removing CI tests for python 3.3

Py.test no longer supports python 3.3:
https://docs.pytest.org/en/latest/changelog.html#pytest-3-3-0-2017-11-23

* Add tests on python v3.8

* Test Python v3.7 instead

v3.8 has no final release yet

* Change dist from trusty to xenial

.travis.yml

@@ -1,13 +1,11 @@
 # enable linux 14.04 environment
 sudo: required
-dist: trusty
-language: python
-
+dist: xenial
 language: python
 
 python:
-  - "3.3"
   - "3.5"
+  - "3.7"
 
 before_install:
   - bash .infrastructure/install_icu.sh