[skip ci] use squash for pull bot

Author: aminya

.github/pull.yml

@@ -2,6 +2,6 @@ version: "1"
 rules:                      # Array of rules
   - base: master            # Required. Target branch
     upstream: atom:master   # Required. Must be in the same fork network.
-    mergeMethod: rebase     # Optional, one of [none, merge, squash, rebase, hardreset], Default: none.
+    mergeMethod: squash     # Optional, one of [none, merge, squash, rebase, hardreset], Default: none.
     mergeUnstable: false    # Optional, merge pull request even when the mergeable_state is not clean. Default: false
-label: ":arrow_heading_down: pull upstream"  # Optional
+label: ":arrow_heading_down: pull upstream"  # Optional