use minimum dependencies rather than specific hashes. (#6)

* use minimum dependencies rather than specific hashes.

* install specific version of pytest and install pylint.

* python 3.7 still not working.

.travis.yml

@@ -6,8 +6,12 @@ python:
   - "3.5"
   - "3.6"
   # - "3.7-dev"
+before_install:
+- "pip install -U pip"
+- "python setup.py install"
 install:
   - pip install tox-travis
-  - pip install tox-pipenv
+  - pip install pytest==3.6.0
+  - pip install pylint
 script:
   - tox