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/groupsmigration/v1/groupsmigration-api.json

@@ -15,7 +15,7 @@
   "description": "Groups Migration Api.",
   "discoveryVersion": "v1",
   "documentationLink": "https://developers.google.com/google-apps/groups-migration/",
-  "etag": "\"-iA1DTNe4s-I6JZXPt1t1Ypy8IU/VuTEwIALyLFp_kdXuKsAPvednIM\"",
+  "etag": "\"Zkyw9ACJZUvcYmlFaKGChzhmtnE/zGiP_i-sBYNkHvFXparCaw6gPyY\"",
   "icons": {
     "x16": "https://www.google.com/images/icons/product/discussions-16.gif",
     "x32": "https://www.google.com/images/icons/product/discussions-32.gif"
@@ -60,12 +60,12 @@
       "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"
     }