Travis: try to fix config for pypy

Author: dnozay

.travis.yml

@@ -1,4 +1,12 @@
 language: python
+python:
+  - '2.7'
+  - '3.4'
+  - '3.5'
+  - '3.6'
+  - '3.7'
+  - 'pypy2.7-6.0'
+  - 'pypy3.5-6.0'
 
 matrix:
   include:
@@ -20,12 +28,16 @@ matrix:
       env: TOXENV=py37
     - os: linux
       dist: xenial
-      python: pypy
+      python: pypy2.7-6.0
       env: TOXENV=pypy
+      services:
+        - docker
     - os: linux
       dist: xenial
-      python: pypy3
+      python: pypy3.5-6.0
       env: TOXENV=pypy3
+      services:
+        - docker
 
 before_install:
   - python --version