Bump go version to 1.11.13

[LucasRoesler]

• 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.

Motivation and Context

• I have raised an issue to propose this change (required)

Resolves #78

How Has This Been Tested?

Built the image and then exec-ing the binary using

docker build -t openfaas/of-watchdog:build .
docker run --rm -it openfaas/of-watchdog:build ./of-watchdog -h

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have signed-off my commits with git commit -s
• I have added tests to cover my changes.
• All new and existing tests passed.

[alexellis]

LGTM