bionic doesn't have python2.6... sticking with trusty and downgrading to python3.6

.travis.yml

@@ -1,5 +1,5 @@
 sudo: required
-dist: bionic
+dist: trusty
 
 notifications:
   email: false
@@ -37,7 +37,7 @@ matrix:
   include:
     # Debops tests.
     # 2.9.6; 3.7 -> 2.7
-    - python: "3.7"
+    - python: "3.6"
       env: MODE=debops_common VER=2.9.6
     # 2.8.3; 3.6 -> 2.7
     - python: "3.6"
@@ -53,7 +53,7 @@ matrix:
     # ansible_mitogen tests.
 
     # 2.9.6 -> {debian, centos6, centos7}
-    - python: "3.7"
+    - python: "3.6"
       env: MODE=ansible VER=2.9.6
     # 2.8.3 -> {debian, centos6, centos7}
     - python: "3.6"
@@ -81,7 +81,7 @@ matrix:
     # 2.7 -> 2.6
     #- python: "2.7"
       #env: MODE=mitogen DISTRO=centos6
-    - python: "3.7"
+    - python: "3.6"
       env: MODE=mitogen DISTRO=centos7
     # 2.6 -> 2.7
     - python: "2.6"