replace `golang.org/x/net/context` to `context`

[binary111]

vendor/github.com/grpc-ecosystem/grpc-opentracing/go/otgrpc/client.go:29:9: cannot use func literal (type func("service/vendor/golang.org/x/net/context".Context, string, interface {}, interface {}, *grpc.ClientConn, grpc.UnaryInvoker, ...grpc.CallOption) error) as type grpc.UnaryClientInterceptor in return argument

As of Go 1.7 this package(golang.org/x/net/context) is available in the standard library under the name context. https://golang.org/pkg/context. So we should replace all golang.org/x/net/context with context.