chore(deps): update dependency nyc to v13 (#285)

Author: renovate[bot]

package.json

@@ -95,7 +95,7 @@
     "intelli-espower-loader": "^1.0.1",
     "jsdoc": "^3.5.5",
     "mocha": "^5.0.0",
-    "nyc": "^12.0.2",
+    "nyc": "^13.0.0",
     "power-assert": "^1.4.4",
     "prettier": "^1.7.4",
     "proxyquire": "^2.0.0",

samples/package.json

@@ -17,7 +17,7 @@
     "@google-cloud/nodejs-repo-tools": "^2.2.3",
     "ava": "^0.25.0",
     "doctoc": "^1.3.1",
-    "nyc": "^12.0.0",
+    "nyc": "^13.0.0",
     "proxyquire": "^2.0.0",
     "sinon": "^6.0.0"
   },