travis: use correct python version

.travis.yml

 language: python
-python:
-  - "2.7"
-  - "3.5"
-  - "3.6"
-  - "3.7"
 sudo: required
 compiler:
   - gcc
@@ -24,14 +19,28 @@ jobs:
   include:
     - dist: bionic
       env: BUILDTYPE=Release
+      python: 2.7
+    - dist: bionic
+      env: BUILDTYPE=Release
+      python: 3.6
     - dist: bionic
       env: BUILDTYPE=Debug
+      python: 2.7
+    - dist: bionic
+      env: BUILDTYPE=Debug
+      python: 3.6
     - dist: xenial
       env: BUILDTYPE=Release
+      python: 2.7
     - dist: xenial
-      env: BUILDTYPE=Debug
+      env: BUILDTYPE=Release
+      python: 3.5
     - dist: trusty 
-      env: BUILDTYPE=Debug
+      env: BUILDTYPE=Release
+      python: 2.7
+    - dist: trusty 
+      env: BUILDTYPE=Release
+      python: 3.4
 
 notifications:
   email: