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/language/v1/language-api.json

@@ -236,7 +236,7 @@
       }
     }
   },
-  "revision": "20180226",
+  "revision": "20180322",
   "rootUrl": "https://language.googleapis.com/",
   "schemas": {
     "AnalyzeEntitiesRequest": {
@@ -514,7 +514,7 @@
           "type": "number"
         },
         "name": {
-          "description": "The name of the category representing the document.",
+          "description": "The name of the category representing the document, from the [predefined\ntaxonomy](/natural-language/docs/categories).",
           "type": "string"
         }
       },

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

@@ -588,7 +588,9 @@ type ClassificationCategory struct {
 	// the classifier is that this category represents the given text.
 	Confidence float64 `json:"confidence,omitempty"`
 
-	// Name: The name of the category representing the document.
+	// Name: The name of the category representing the document, from the
+	// [predefined
+	// taxonomy](/natural-language/docs/categories).
 	Name string `json:"name,omitempty"`
 
 	// ForceSendFields is a list of field names (e.g. "Confidence") to

vendor/google.golang.org/api/language/v1beta2/language-api.json

@@ -236,7 +236,7 @@
       }
     }
   },
-  "revision": "20180226",
+  "revision": "20180322",
   "rootUrl": "https://language.googleapis.com/",
   "schemas": {
     "AnalyzeEntitiesRequest": {
@@ -514,7 +514,7 @@
           "type": "number"
         },
         "name": {
-          "description": "The name of the category representing the document.",
+          "description": "The name of the category representing the document, from the [predefined\ntaxonomy](/natural-language/docs/categories).",
           "type": "string"
         }
       },

vendor/google.golang.org/api/language/v1beta2/language-gen.go

@@ -589,7 +589,9 @@ type ClassificationCategory struct {
 	// the classifier is that this category represents the given text.
 	Confidence float64 `json:"confidence,omitempty"`
 
-	// Name: The name of the category representing the document.
+	// Name: The name of the category representing the document, from the
+	// [predefined
+	// taxonomy](/natural-language/docs/categories).
 	Name string `json:"name,omitempty"`
 
 	// ForceSendFields is a list of field names (e.g. "Confidence") to