Release v3 - with full go module support

[dnephin]

To properly support go modules we need a new import path. Following https://github.com/golang/go/wiki/Modules#releasing-modules-v2-or-higher the correct way to do that seems to be to release a new major version.

According to those docs this will be supported by the latest patch release of go1.9 and go1.10 as well.

Closes #156

[codecov]

Codecov Report

Merging #140 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #140   +/-   ##
=======================================
  Coverage   83.16%   83.16%           
=======================================
  Files          29       29           
  Lines        2038     2038           
=======================================
  Hits         1695     1695           
  Misses        236      236           
  Partials      107      107

Impacted Files	Coverage Δ
fs/ops.go	72.03% <ø> (ø)	⬆️
env/env.go	89.65% <ø> (ø)	⬆️
fs/manifest.go	86.66% <ø> (ø)	⬆️
golden/golden.go	90.16% <ø> (ø)	⬆️
fs/report.go	84.17% <ø> (ø)	⬆️
fs/path.go	86.9% <ø> (ø)	⬆️
assert/cmp/compare.go	88.77% <ø> (ø)	⬆️
internal/format/diff.go	88.88% <ø> (ø)	⬆️
assert/cmp/result.go	71.42% <ø> (ø)	⬆️
icmd/command.go	85.29% <ø> (ø)	⬆️
... and 4 more

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[codecov]

Codecov Report

Merging #140 into master will decrease coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #140      +/-   ##
==========================================
- Coverage    82.8%   82.72%   -0.09%     
==========================================
  Files          29       29              
  Lines        2030     2020      -10     
==========================================
- Hits         1681     1671      -10     
  Misses        242      242              
  Partials      107      107

Impacted Files	Coverage Δ
env/env.go	89.65% <ø> (ø)	⬆️
internal/format/diff.go	88.88% <ø> (ø)	⬆️
golden/golden.go	86.66% <ø> (-2.43%)	⬇️
icmd/command.go	85.18% <ø> (ø)	⬆️
assert/cmp/compare.go	88.88% <ø> (ø)	⬆️
fs/ops.go	66.1% <ø> (ø)	⬆️
fs/manifest.go	86.66% <ø> (ø)	⬆️
assert/result.go	72.91% <ø> (ø)	⬆️
fs/report.go	84.17% <ø> (ø)	⬆️
fs/path.go	84.52% <ø> (ø)	⬆️
... and 5 more

---

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Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d9c679c...4c906a1. Read the comment docs.

[vdemeester]

I would prefer "Major subdirectory:" solution as it provides the greatest backwards compatibility (to support ~1.8). That said, I feels it would be way more work for not too much… so, I think SGTM but I'm not sure I see all the implication (as, running dep ensure -update github.com/gotestyourself/gotest.tools, what will it do ?).

Any way, we're doing what the go docs says, and we gotta start somewhere so… LGTM :wink: