profiler: only build proftest with go1.7 or higher
golang.org/x/build/kubernetes does not support go1.6
// Package proftest contains test helpers for profiler agent integration tests.
// This package is experimental.
+// golang.org/x/build/kubernetes/dialer.go imports "context" package (rather
+// than "golang.org/x/net/context") and that does not exist in Go 1.6 or
+// earlier.
+// +build go1.7
+
package proftest
import (
