Fix codecov on Travis CI

The coverage report needs to be explicitly created with arguments passed
in to pytest-cov

.travis.yml

@@ -31,4 +31,4 @@ script:
 
 after_success:
     - pip install codecov
-    - codecov
+    - codecov -e TOX_ENV

runtests.py

@@ -93,7 +93,11 @@ if __name__ == "__main__":
         except ValueError:
             pass
         else:
-            pytest_args = ['--cov', 'rest_framework'] + pytest_args
+            pytest_args = [
+                '--cov-report',
+                'xml',
+                '--cov',
+                'rest_framework'] + pytest_args
 
         if first_arg.startswith('-'):
             # `runtests.py [flags]`