Fix .travis.yml #1

.travis.yml

@@ -1,14 +1,12 @@
 language: python
 python:
- - '3.5'
  - '3.6'
- - '3.7'
 env:
  - TOX_ENV=py35
  - TOX_ENV=py36
  - TOX_ENV=py37
 install:
- - pip install requirements_dev.txt
+ - pip install -r requirements_dev.txt
 before_script:
  - coverage erase
 script: