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/appengine/v1alpha/appengine-api.json

@@ -547,7 +547,7 @@
         "locations": {
           "methods": {
             "get": {
-              "description": "Get information about a location.",
+              "description": "Gets information about a location.",
               "flatPath": "v1alpha/apps/{appsId}/locations/{locationsId}",
               "httpMethod": "GET",
               "id": "appengine.apps.locations.get",
@@ -705,7 +705,7 @@
       }
     }
   },
-  "revision": "20180307",
+  "revision": "20180320",
   "rootUrl": "https://appengine.googleapis.com/",
   "schemas": {
     "AuthorizedCertificate": {
@@ -789,6 +789,17 @@
       },
       "type": "object"
     },
+    "CreateVersionMetadataV1": {
+      "description": "Metadata for the given google.longrunning.Operation during a google.appengine.v1.CreateVersionRequest.",
+      "id": "CreateVersionMetadataV1",
+      "properties": {
+        "cloudBuildId": {
+          "description": "The Cloud Build ID if one was created as part of the version create. @OutputOnly",
+          "type": "string"
+        }
+      },
+      "type": "object"
+    },
     "CreateVersionMetadataV1Alpha": {
       "description": "Metadata for the given google.longrunning.Operation during a google.appengine.v1alpha.CreateVersionRequest.",
       "id": "CreateVersionMetadataV1Alpha",
@@ -1090,6 +1101,9 @@
       "description": "Metadata for the given google.longrunning.Operation.",
       "id": "OperationMetadataV1",
       "properties": {
+        "createVersionMetadata": {
+          "$ref": "CreateVersionMetadataV1"
+        },
         "endTime": {
           "description": "Time that this operation completed.@OutputOnly",
           "format": "google-datetime",