Relax low coverage trealdshold

Change-Id: Id5579ead3ded20987f0e4d4e950054e9da8768b4

.coveragerc

@@ -5,3 +5,5 @@ omit = tobiko/tests/*
 
 [report]
 ignore_errors = True
+exclude_lines =
+    raise NotImplementedError

tox.ini

@@ -48,7 +48,7 @@ commands =
     coverage combine
     coverage html -d cover
     coverage xml -o cover/coverage.xml
-    coverage report --fail-under=50 --skip-covered
+    coverage report --fail-under=40 --skip-covered
     find . -type f -name ".coverage.*" -delete
 whitelist_externals =
     find