Go module/1.11.x support, vendored deps, cttestsrv api fixes.

[cpu]

• Fixes the cttestsrv API breakage with Trillian master from time source refactoring.
• Updates the project/CI to use Go 1.11.x
• Configures a ct-woodpecker module
• Vendors dependencies[0]
• Removes errcheck[1] from CI
• Updates CI to use vendored deps throughout
• Fixes a few typos flagged by Goreportcard

Resolves https://github.com/letsencrypt/ct-woodpecker/issues/72 and https://github.com/letsencrypt/ct-woodpecker/issues/73 and fixes CI.

[0]: Notably this pins Trillian to d0c8d5f - the tip of master at the time of writing. We're using changes that aren't in the latest v1.2.1 tag. Similarly, Trillian at tip of master requires us to pin golang/protobuf at 1d3f30b - the tip of master for that project at the time of writing. Trillian is using changes in this project that aren't in the latest v1.2.0 tag.

[1]: Errcheck doesn't work with Go modules and our vendored deps. Adding it back is blocked on https://github.com/kisielk/errcheck/issues/155 Personally I think the stability improvements of vendored dependencies merits removing errcheck from Travis and the cron builds. We do still get errcheck coverage per-PR with GolangCI.

[coveralls]

Pull Request Test Coverage Report for Build 484

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.3%) to 67.515%

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Totals
Change from base Build 365:	0.3%
Covered Lines:	769
Relevant Lines:	1139

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💛 - Coveralls