logging-winston: make winston a peerDependency (#2453)

Users of this library need to depend on winston directly. There are
scenarios where the user, or other libraries may depend on a semver
incompatible version of winston. Not only does it end up installing an
unnecessary copy of winston; it may also lead to our transport not
being visible in `winston.transports` that the user has visibility to.

packages/logging-winston/package.json

@@ -32,12 +32,15 @@
     "@google-cloud/logging": "^1.0.0",
     "extend": "^3.0.0",
     "is": "^3.2.0",
-    "lodash.mapvalues": "^4.6.0",
-    "winston": "^2.2.0"
+    "lodash.mapvalues": "^4.6.0"
+  },
+  "peerDependencies": {
+    "winston": "^2.x"
   },
   "devDependencies": {
     "mocha": "^3.2.0",
-    "proxyquire": "^1.7.10"
+    "proxyquire": "^1.7.10",
+    "winston": "^2.2.0"
   },
   "scripts": {
     "publish-module": "node ../../scripts/publish.js logging-winston",