Fix error in travis.yml with the new Pipfile.

.travis.yml

@@ -1,11 +1,10 @@
 language: python
 python:
-  - 3.5
   - 3.6
 
 install:
-  - "pip install -r requirements.txt"
-  - "pip install -r requirements.dev.txt"
+  - "pipenv install"
+  - "pipenv install --dev"
 
 script:
   # Run the test suit

Pipfile

@@ -14,3 +14,7 @@ crayons = "*"
 
 [dev-packages]
 
+pep8 = "*"
+coverage = "*"
+coveralls = "*"
+mypy = "*"

requirements.dev.txt

@@ -1,4 +0,0 @@
-pep8
-coverage
-coveralls
-mypy