Added c8 --all to surface true members-api coverage

refs: https://github.com/TryGhost/Toolbox/issues/203

- Without the all flag only files touched by tests are counted
- This shows our unit-testing picture more clearly
- Will get this change rolled out across all repos and packages ASAP, but for now this one seemed most important

ghost/members-api/package.json

@@ -7,7 +7,7 @@
   "main": "index.js",
   "scripts": {
     "dev": "echo \"Implement me!\"",
-    "test": "NODE_ENV=testing c8 --reporter text --reporter cobertura mocha --reporter dot './test/**/*.test.js'",
+    "test": "NODE_ENV=testing c8 --all --reporter text --reporter cobertura mocha --reporter dot './test/**/*.test.js'",
     "lint": "eslint '**/*.js'",
     "posttest": "yarn lint"
   },