Remove Python 3.3 for travis-ci often failing, add Python3.7.

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index 7057bb6..ec8e9e5 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,9 +1,9 @@
 language: python
 python:
-- '3.3'
 - '3.4'
 - '3.5'
 - '3.6'
+- '3.7'
 install:
 - pip install -r requirements.txt
 - pip install coveralls