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/dataflow/v1b3/dataflow-api.json

@@ -1554,7 +1554,7 @@
       }
     }
   },
-  "revision": "20180314",
+  "revision": "20180413",
   "rootUrl": "https://dataflow.googleapis.com/",
   "schemas": {
     "ApproximateProgress": {
@@ -1658,6 +1658,10 @@
           "description": "The time this event was emitted to indicate a new target or current\nnum_workers value.",
           "format": "google-datetime",
           "type": "string"
+        },
+        "workerPool": {
+          "description": "A short and friendly name for the worker pool this event refers to,\npopulated from the value of PoolStageRelation::user_pool_name.",
+          "type": "string"
         }
       },
       "type": "object"
@@ -3146,6 +3150,10 @@
           "description": "A struct value describing properties of a distribution of numeric values.",
           "type": "any"
         },
+        "gauge": {
+          "description": "A struct value describing properties of a Gauge.\nMetrics of gauge type show the value of a metric across time, and is\naggregated based on the newest value.",
+          "type": "any"
+        },
         "internal": {
           "description": "Worker-computed aggregate value for internal use by the Dataflow\nservice.",
           "type": "any"