Remove go1.6, go1.7, go1.8 support

[dmarkham]

I'm aware removing go1.6/go1.7 was scheduled for v1.5.0 but given the amount of time that has passed we now have go1.12 so supporting only go1.9+ is older than go supports.

[codecov]

Codecov Report

Merging #1843 into master will increase coverage by 0.08%.
The diff coverage is 87.5%.

@@            Coverage Diff             @@
##           master    #1843      +/-   ##
==========================================
+ Coverage   98.64%   98.73%   +0.08%     
==========================================
  Files          41       38       -3     
  Lines        2144     2138       -6     
==========================================
- Hits         2115     2111       -4     
+ Misses         18       15       -3     
- Partials       11       12       +1

Impacted Files	Coverage Δ
debug.go	95.91% <100%> (ø)	⬆️
render/redirect.go	100% <100%> (ø)	⬆️
context.go	98.36% <100%> (-0.02%)	⬇️
render/json.go	87.65% <100%> (+1.16%)	⬆️
response_writer.go	95.23% <50%> (-4.77%)	⬇️

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[thinkerou]

please see #1631 thanks!