(fix): upgrade object-hash to support async/await syntax

- object-hash v2.0.2 adds/fixes support for async functions
  - object-hash v2 has no breaking changes, just drops testing of
    Node < 6
    - Node 5 was EOL April 2018, and on top of that Node 6
      was EOL April 2019, and Node 8 was EOL December 2019, so this is
      still more tested support than is necessary

- objectHashIgnoreUnknownHack isn't useful for its original purpose to
  workaround this, but is still useful for other types of
  objects/syntaxes that object-hash may not handle yet
  (e.g. async generator functions), so leave the hack in
  - might be good to add a warning though? so that those who used this
    option for async/await can upgrade and remove it (and have a
    stable cache again)

package.json

@@ -52,7 +52,7 @@
     "colors": "1.3.3",
     "graphlib": "2.1.7",
     "lodash": "4.17.15",
-    "object-hash": "1.3.1",
+    "object-hash": "2.0.2",
     "rimraf": "3.0.0",
     "rollup": "^1.26.3 ",
     "rollup-plugin-commonjs": "10.1.0",