gitignore: Add coverage/

Coverage is where PhantomJs outputs the unit test coverage.

Change-Id: I381e479d57dc2226a464d117bfd5a917bd5706dd
Signed-off-by: Gunnar Mills <gmills@us.ibm.com>
diff --git a/.gitignore b/.gitignore
index b36a8c0..3a72cbb 100644
--- a/.gitignore
+++ b/.gitignore
@@ -7,4 +7,5 @@
 *.swp
 .idea
 app/styles/index.css
-yarn.lock
\ No newline at end of file
+yarn.lock
+coverage/