fix: .snyk & package.json to reduce vulnerabilities

The following vulnerabilities are fixed with a Snyk patch:
- https://snyk.io/vuln/SNYK-JS-TREEKILL-536781

.snyk

@@ -1,5 +1,5 @@
 # Snyk (https://snyk.io) policy file, patches or ignores known vulnerabilities.
-version: v1.13.5
+version: v1.14.0
 ignore: {}
 # patches apply the minimum changes required to fix a vulnerability
 patch:
@@ -87,3 +87,6 @@ patch:
         patched: '2019-10-04T01:06:11.331Z'
     - snyk > proxy-agent > pac-proxy-agent > https-proxy-agent:
         patched: '2019-10-04T01:06:11.331Z'
+  SNYK-JS-TREEKILL-536781:
+    - snyk > snyk-sbt-plugin > tree-kill:
+        patched: '2019-12-12T01:01:32.150Z'

package.json

@@ -23,7 +23,7 @@
     "nuxt": "^2.0.0",
     "s-ago": "^1.3.0",
     "sass-loader": "^7.0.3",
-    "snyk": "^1.231.0",
+    "snyk": "^1.258.2",
     "vue": "^2.5.22",
     "vue-lazyload": "^1.2.6",
     "vue-numeral-filter": "^1.1.1",
@@ -41,7 +41,7 @@
     "test:cypress": "cypress run",
     "wait": "wait-on http://localhost:3000",
     "snyk-protect": "snyk protect",
-    "prepublish": "npm run snyk-protect"
+    "prepublish": "yarn run snyk-protect"
   },
   "devDependencies": {
     "@nuxtjs/vuetify": "^1.9.0",