gitignore cover_db

Also MYMETA.json.lock seems to be a possible leftover of Devel::Cover
runs.

Author: eserte

.gitignore

@@ -1,8 +1,10 @@
 /.prove
 /MYMETA.json
+/MYMETA.json.lock
 /MYMETA.yml
 /Makefile
 /Makefile.old
 /blib
 /pm_to_blib
 /var
+/cover_db/