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/runtimeconfig/v1/runtimeconfig-api.json

@@ -220,7 +220,7 @@
       }
     }
   },
-  "revision": "20180108",
+  "revision": "20180416",
   "rootUrl": "https://runtimeconfig.googleapis.com/",
   "schemas": {
     "CancelOperationRequest": {
@@ -317,7 +317,7 @@
     }
   },
   "servicePath": "",
-  "title": "Google Cloud Runtime Configuration API",
+  "title": "Cloud Runtime Configuration API",
   "version": "v1",
   "version_module": true
 }

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

@@ -1,4 +1,4 @@
-// Package runtimeconfig provides access to the Google Cloud Runtime Configuration API.
+// Package runtimeconfig provides access to the Cloud Runtime Configuration API.
 //
 // See https://cloud.google.com/deployment-manager/runtime-configurator/
 //