vendor: update all dependencies

* Update all dependencies
  * Remove all `[[constraint]]` from Gopkg.toml
  * Add in the minimum number of `[[override]]` to build
  * Remove go get of github.com/inconshreveable/mousetrap as it is vendored
  * Update docs with new policy on constraints

vendor/google.golang.org/api/plusdomains/v1/plusdomains-api.json

@@ -41,7 +41,7 @@
   "description": "Builds on top of the Google+ platform for Google Apps Domains.",
   "discoveryVersion": "v1",
   "documentationLink": "https://developers.google.com/+/domains/",
-  "etag": "\"-iA1DTNe4s-I6JZXPt1t1Ypy8IU/uFI9OtaBYnIYFMdaxO5kg4qsSVg\"",
+  "etag": "\"Zkyw9ACJZUvcYmlFaKGChzhmtnE/QN-lMFT7XaFWRpPfYwaRCAFMlw8\"",
   "icons": {
     "x16": "http://www.google.com/images/icons/product/gplus-16.png",
     "x32": "http://www.google.com/images/icons/product/gplus-32.png"
@@ -86,12 +86,12 @@
       "type": "boolean"
     },
     "quotaUser": {
-      "description": "Available to use for quota purposes for server-side applications. Can be any arbitrary string assigned to a user, but should not exceed 40 characters. Overrides userIp if both are provided.",
+      "description": "An opaque string that represents a user for quota purposes. Must not exceed 40 characters.",
       "location": "query",
       "type": "string"
     },
     "userIp": {
-      "description": "IP address of the site where the request originates. Use this if you want to enforce per-user limits.",
+      "description": "Deprecated. Please use quotaUser instead.",
       "location": "query",
       "type": "string"
     }
@@ -852,7 +852,7 @@
       }
     }
   },
-  "revision": "20180127",
+  "revision": "20180424",
   "rootUrl": "https://www.googleapis.com/",
   "schemas": {
     "Acl": {