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/cloudtrace/v1/cloudtrace-api.json

@@ -261,7 +261,7 @@
       }
     }
   },
-  "revision": "20180312",
+  "revision": "20180430",
   "rootUrl": "https://cloudtrace.googleapis.com/",
   "schemas": {
     "Empty": {
@@ -279,7 +279,7 @@
           "type": "string"
         },
         "traces": {
-          "description": "List of trace records returned.",
+          "description": "List of trace records as specified by the view parameter.",
           "items": {
             "$ref": "Trace"
           },

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

@@ -129,7 +129,7 @@ type ListTracesResponse struct {
 	// retrieving additional traces.
 	NextPageToken string `json:"nextPageToken,omitempty"`
 
-	// Traces: List of trace records returned.
+	// Traces: List of trace records as specified by the view parameter.
 	Traces []*Trace `json:"traces,omitempty"`
 
 	// ServerResponse contains the HTTP response code and headers from the