Go 1.10.4 build fails with undef imports in http2 pkg

[chrishenzie]

Recent Travis CI builds for Go 1.10.4 fail with the following errors:

# golang.org/x/net/http2
../../golang.org/x/net/http2/client_conn_pool.go:305:6: undefined: errors.Is
../../golang.org/x/net/http2/server.go:234:72: undefined: tls.VersionTLS13

See here and here for sample failures.

[gnufied]

It should be fixed by #435 when it merges.

[chrishenzie]

Fixed, thank you!