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/people/v1/people-api.json

@@ -596,7 +596,7 @@
       }
     }
   },
-  "revision": "20180303",
+  "revision": "20180405",
   "rootUrl": "https://people.googleapis.com/",
   "schemas": {
     "Address": {
@@ -897,7 +897,7 @@
           "type": "integer"
         },
         "month": {
-          "description": "Month of year. Must be from 1 to 12.",
+          "description": "Month of year. Must be from 1 to 12, or 0 if specifying a date without a\nmonth.",
           "format": "int32",
           "type": "integer"
         },

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

@@ -668,7 +668,9 @@ type Date struct {
 	// if specifying a year/month where the day is not significant.
 	Day int64 `json:"day,omitempty"`
 
-	// Month: Month of year. Must be from 1 to 12.
+	// Month: Month of year. Must be from 1 to 12, or 0 if specifying a date
+	// without a
+	// month.
 	Month int64 `json:"month,omitempty"`
 
 	// Year: Year of date. Must be from 1 to 9999, or 0 if specifying a date