Remove screeps-server-mockup dependency (#113)

Remove "screeps-server-mockup": "^1.4.3" since it's deprecated and causing build errors for new users of the starter template.

https://www.npmjs.com/package/screeps-server-mockup

package.json

@@ -48,7 +48,6 @@
     "rollup-plugin-nodent": "^0.2.2",
     "rollup-plugin-screeps": "^0.1.2",
     "rollup-plugin-typescript2": "^0.16.1",
-    "screeps-server-mockup": "^1.4.3",
     "sinon": "^6.3.5",
     "sinon-chai": "^3.2.0",
     "ts-node": "^7.0.1",