travis: fix 'Directory u'../' is not installable'

Author: zhangchunlin

.travis.yml

@@ -3,5 +3,5 @@ python:
   - "3.6"
   - "3.7"
 sudo: required
-before_install: sudo pip install -r tests/requirements.txt
+before_install: cd tests;sudo pip install -r requirements.txt
 script: cd tests;nosetests --with-doctest