Fix GOPROXY problem

[mhutchinson]

The GOPROXY has begun to return 410's for some of our dependencies.

This PR attempts to fix this by updating to fresher dependencies, more of them being explicit versions, and therefore a higher likelihood of being found by the proxy.

This is a clone of #1336 because Gary is on holiday and Travis failed the build of that branch so we can't submit.

[jtoohill]

It looks like Travis failed on a module that we aren't depending on any more?

go: github.com/russross/blackfriday/[email protected]+incompatible: unexpected status (https://proxy.golang.org/github.com/russross/blackfriday/v2/@v/v2.0.1+incompatible.info): 410 Gone
go: error loading module requirements
The command "eval go get -v -t ./... " failed. Retrying, 2 of 3.
g

Approving anyways. It'd be good to update README.md with instructions for how to maintain go.mod, but maybe Gary should do that when he gets back.

[mhutchinson]

This all builds fine on my machine. But I can get it to list the problematic import of blackfriday by doing the following:
go clean -cache -modcache
go get -v ./... # This updates go.mod
go list -m all

The problem seems to be that Trillian at 1.3.0 has some dodgy dependencies, including the "github.com/russross/blackfriday/v2 v2.0.1+incompatible" (which cannot be fetched - https://proxy.golang.org/github.com/russross/blackfriday/v2/@v/v2.0.1+incompatible.info) that I'm growing so fond of. I don't think this can be worked around except without pushing a new Trillian release that includes google/trillian#1785.

For future reference, the way I diagnosed the reason that the +incompatible dependency snuck in is via the very handy graph feature:
go mod graph | grep blackfriday

[codecov]

Codecov Report

Merging #1337 into master will decrease coverage by 0.03%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1337      +/-   ##
==========================================
- Coverage   30.31%   30.28%   -0.04%     
==========================================
  Files          48       48              
  Lines        3866     3867       +1     
==========================================
- Hits         1172     1171       -1     
- Misses       2512     2514       +2     
  Partials      182      182

Impacted Files	Coverage Δ
impl/integration/env.go	0% <0%> (ø)	⬆️
core/client/client.go	28.28% <0%> (-0.66%)	⬇️

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