travis: travis use virtualenv, so don't need sudo

Author: zhangchunlin

.travis.yml

@@ -2,6 +2,5 @@ language: python
 python:
   - "3.6"
   - "3.7"
-sudo: required
-before_install: cd tests;sudo pip install -r requirements.txt
+before_install: cd tests;pip install -r requirements.txt
 script: cd tests;nosetests --with-doctest