Bump go version to 1.11.13

**What**
- Using the base golang:1.11 docker image so that we can easily track
security patches for go 1.11. In particular this includes go 1.11.13,
which address

 https://groups.google.com/forum/#!topic/golang-announce/65QixT3tcmg

> net/http: Denial of Service vulnerabilities in the HTTP/2 implementation
>
> net/http and golang.org/x/net/http2 servers that accept direct connections
> from untrusted clients could be remotely made to allocate an unlimited amount
> of memory, until the program crashes. Servers will now close connections if
> the send queue accumulates too many control messages.
>
> net/url: parsing validation issue
>
> url.Parse would accept URLs with malformed hosts, such that the Host field
> could have arbitrary suffixes that would appear in neither Hostname() nor
> Port(), allowing authorization bypasses in certain applications. Note that URLs
> with invalid, not numeric ports will now return an error from url.Parse.

Signed-off-by: Lucas Roesler <roesler.lucas@gmail.com>

Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.10
+FROM golang:1.11
 
 RUN mkdir -p /go/src/github.com/openfaas-incubator/of-watchdog
 WORKDIR /go/src/github.com/openfaas-incubator/of-watchdog