chore(ci-coverage): Pin coverage 4.5.4 #20646 (#20647)

* chore(ci-coverage): Pin coveralls 4.5.4 #20646

Signed-off-by: mathieu.brunot <mathieu.brunot@monogramm.io>

* chore: Pin coverage

Signed-off-by: mathieu.brunot <mathieu.brunot@monogramm.io>
diff --git a/.travis.yml b/.travis.yml
index 365eb67..213445b 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -77,5 +77,6 @@
   - bench --site test_site reinstall --yes
 
 after_script:
+  - pip install coverage==4.5.4
   - pip install python-coveralls
   - coveralls -b apps/erpnext -d ../../sites/.coverage