improve coverage configuration

Use dedicated directory - `.tests_reports/` - for all tests reports,
including `coverage` html, xml and json reports.
Update `coverage` configuration to:

+ measure `branch` coverage, refer to documentation:
  https://coverage.readthedocs.io/en/coverage-5.0.4/branch.html
+ show only not covered files and lines
+ sort report's results by coverage value

.gitignore

@@ -199,3 +199,4 @@ fabric.properties
 ### Custom ###
 ex.py
 *.sqlite3
+.tests_reports/

.travis.yml

@@ -32,7 +32,7 @@ script:
 
 after_success:
   - pip install coveralls
-  - coveralls
+  - coveralls --rcfile=setup.cfg
 
 notifications:
   email:

setup.cfg

@@ -88,9 +88,25 @@ addopts =
 # Why do we exclude this file from coverage?
 # Because coverage is not calculated correctly for pytest plugins.
 # And we completely test it anyway.
+branch = True
+data_file = .tests_reports/.coverage
 omit =
   django_test_migrations/contrib/pytest_plugin.py
 
+[coverage:report]
+skip_covered = True
+show_missing = True
+sort = Cover
+
+[coverage:xml]
+output = .tests_reports/coverage.xml
+
+[coverage:json]
+output = .tests_reports/coverage.json
+
+[coverage:html]
+directory = .tests_reports/htmlcov
+
 
 [mypy]
 # mypy configurations: http://bit.ly/2zEl9WI