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/abusiveexperiencereport/v1/abusiveexperiencereport-api.json

@@ -164,11 +164,11 @@
       }
     }
   },
-  "revision": "20180129",
+  "revision": "20180419",
   "rootUrl": "https://abusiveexperiencereport.googleapis.com/",
   "schemas": {
     "SiteSummaryResponse": {
-      "description": "Response message for GetSiteSummary.",
+      "description": "Response message for GetSiteSummary.\nDo not confuse with same message in google.ads.experiencereport.v1",
       "id": "SiteSummaryResponse",
       "properties": {
         "abusiveStatus": {
@@ -244,7 +244,7 @@
     }
   },
   "servicePath": "",
-  "title": "Google Abusive Experience Report API",
+  "title": "Abusive Experience Report API",
   "version": "v1",
   "version_module": true
 }

vendor/google.golang.org/api/abusiveexperiencereport/v1/abusiveexperiencereport-gen.go

@@ -1,4 +1,4 @@
-// Package abusiveexperiencereport provides access to the Google Abusive Experience Report API.
+// Package abusiveexperiencereport provides access to the Abusive Experience Report API.
 //
 // See https://developers.google.com/abusive-experience-report/
 //
@@ -97,6 +97,7 @@ type ViolatingSitesService struct {
 }
 
 // SiteSummaryResponse: Response message for GetSiteSummary.
+// Do not confuse with same message in google.ads.experiencereport.v1
 type SiteSummaryResponse struct {
 	// AbusiveStatus: The status of the site reviewed for the abusive
 	// experiences.