chore: add dependency on @babel/traverse

Author: michaelfig

packages/bundle-source/package.json

@@ -23,6 +23,7 @@
     "@agoric/harden": "^0.0.8",
     "@agoric/transform-eventual-send": "^1.3.1",
     "@babel/generator": "^7.6.4",
+    "@babel/traverse": "^7.8.3",
     "@rollup/plugin-commonjs": "^11.0.2",
     "@rollup/plugin-node-resolve": "^7.1.1",
     "acorn": "^7.1.0",