specify the correct distros for each python version

Author: akhmerov

.travis.yml

@@ -1,9 +1,15 @@
 language: python
-dist: bionic
-python:
-  - "3.5"
-  - "3.6"
-  - "3.7"
+matrix:
+  include:
+    - name: "Python 3.5"
+      dist: xenial
+      python: 3.5
+    - name: "Python 3.6"
+      dist: bionic
+      python: 3.6
+    - name: "Python 3.7"
+      dist: cosmic
+      python: 3.7
 install:
   - pip install .
 script: