.coveragerc: use include instead of multiple sources (joke2k#183)

This will generate a better coverage.xml file, which makes it easier for
codecov hopefully.

Old:

    <sources>
            <source>…/Vcs/django-configurations/configurations</source>
            <source>…/Vcs/django-configurations/tests</source>
    </sources>
    <packages>
            <package branch-rate="0.7178" complexity="0" line-rate="0.8902" name=".">

New:
    <sources>
            <source>…/Vcs/django-configurations</source>
    </sources>
    <packages>
            <package branch-rate="0.712" complexity="0" line-rate="0.8126" name="configurations">

Fixes jazzband/django-configurations@d364802#commitcomment-24826518.

.coveragerc

@@ -1,3 +1,4 @@
 [run]
-source = configurations,tests
+source = .
+include = configurations/*,tests/*
 branch = 1