Set arc.land.onto.default to dev

Summary:
Since all development goes to `dev` branch, this sets the `arc land` to
use `dev` branch as the destination. Typing `arc land` should now behave
the same as `arc land --onto dev`

Note that "arc.land.onto.default" only takes a branch name. The name
will be prepended with remote name, which is `origin` by default. So,
the commit will land by default onto `origin/dev`.

Reviewers: buda

Reviewed By: buda

Subscribers: pullbot

Differential Revision: https://phabricator.memgraph.io/D89

.arcconfig

@@ -2,5 +2,6 @@
   "project_id" : "memgraph",
   "conduit_uri" : "https://phabricator.memgraph.io",
   "phabricator_uri" : "https://phabricator.memgraph.io",
-  "git.default-relative-commit": "origin/dev"
+  "git.default-relative-commit": "origin/dev",
+  "arc.land.onto.default": "dev"
 }