Removed unnecessary install step from .travis.yml

Author: Nathan Lee

.travis.yml

@@ -6,9 +6,6 @@ python:
   - "3.6"
   - "3.6-dev"
   - "3.7"
-# command to install dependencies
-install:
-  - pip install -r requirements.txt
 # command to run tests
 script:
   - pytest