Add ignore files related to coverage to .gitignore

.coverage and cover are generated by tox's cover task. So we should
ignore them.

Change-Id: Ica14d566fcd7bd31fea87471178db882add7813a
diff --git a/.gitignore b/.gitignore
index 0f4880f..8d2b281 100644
--- a/.gitignore
+++ b/.gitignore
@@ -15,3 +15,5 @@
 dist
 build
 .testrepository
+.coverage
+cover/