Check "x/net/context" with go vet like "context" (#1490)

* Check "x/net/context" with `go vet` like "context"

Includes a signficant revamp of .travis.yml to execute a separate script for
all of the things we check before allowing a merge.

* fix bug in benchmain

benchmark/benchmain/main.go

@@ -259,7 +259,7 @@ func init() {
 		runMode[0] = true
 		runMode[1] = true
 	default:
-		log.Fatalf("Unknown workloads setting: %v (want %v, %v, or %v)",
+		log.Fatalf("Unknown workloads setting: %v (want one of: %v)",
 			workloads, strings.Join(allWorkloads, ", "))
 	}
 	switch compressorMode {