Use https instead of git for bitbucket

This allows tools/external_updater to successfully check for new
versions (and find no newer versions).

Change-Id: I8ea6290bd56aee38b170ac3b86c05f4be5ddcb28
diff --git a/METADATA b/METADATA
index 344dd49..06e3d85 100644
--- a/METADATA
+++ b/METADATA
@@ -5,7 +5,7 @@
 third_party {
   url {
     type: GIT
-    value: "git://bitbucket.org/creachadair/shell.git"
+    value: "https://bitbucket.org/creachadair/shell.git"
   }
   version: "v0.0.6"
   last_upgrade_date { year: 2019 month: 06 day: 03 }