fix: package.json to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/npm:tunnel-agent:20170305

package.json

@@ -27,7 +27,7 @@
   },
   "homepage": "https://github.com/piranna/easy-coveralls#readme",
   "dependencies": {
-    "coveralls": "^2.11.11",
+    "coveralls": "^3.0.0",
     "fs-extra": "^0.30.0",
     "jscoverage": "github:piranna/jscoverage",
     "mocha-lcov-reporter": "^1.2.0"