[REF] .t2d.yml: Fix runbot red

Docker image used from runbot is too old
So it is using python3.7 as last python version installed
But only 4.5.4 coverage version is supported by this version of py

.t2d.yml

@@ -5,10 +5,10 @@ cache:
     - $HOME/.cache/pre-commit
 
 python:
-  - "3.8"
+  - "3.7"
 
 addons:
-  postgresql: "9.6"
+  postgresql: "11"
   apt:
     packages:
       - expect-dev # provides unbuffer utility
@@ -27,13 +27,13 @@ jobs:
 env:
   global:
     - VERSION="15.0" TESTS="0" LINT_CHECK="0" MAKEPOT="0"
-    - MQT_DEP=PIP
 
 install:
   - git clone --depth=1 https://github.com/OCA/maintainer-quality-tools.git
     ${HOME}/maintainer-quality-tools
   - export PATH=${HOME}/maintainer-quality-tools/travis:${PATH}
   - travis_install_nightly
+  - pip install -U coverage==4.5.4
 
 script:
   - travis_run_tests