Change review policy for PRs that don't target master

[travisbrown]

I originally proposed this policy change on Gitter:

I'd like to propose that we lower the review requirement for non-master PRs in Cats.

Specifically Gagandeep has put a huge amount of effort into backporting 2.1 changes for Scala 2.11, and these contributions have been getting merged very slowly: https://github.com/typelevel/cats/pulls?q=is%3Apr+author%3Agagandeepkalra+backport

There are currently 10 open. I've reviewed all of them, and in a few cases we've gone through several iterations to get them right.

I know backport branches like this are a low priority for most maintainers (including me), but they can be very important for contributors like Gagandeep, and I think it would be reasonable to say PRs against these branches only need one non-author +1 to merge.

…but on second thought it seems easiest just to make the change as a PR and we can discuss it here.

[codecov-io]

Codecov Report

Merging #3354 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3354   +/-   ##
=======================================
  Coverage   93.33%   93.33%           
=======================================
  Files         378      378           
  Lines        7692     7692           
  Branches      206      206           
=======================================
  Hits         7179     7179           
  Misses        513      513           

Flag	Coverage Δ
#scala_version_212	93.38% <ø> (ø)
#scala_version_213	93.11% <ø> (ø)

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[kailuowang]

I think it makes sense.

[rossabaker]

👍 I've also been trying to help out on those, but my OSS time ebbs and flows, and this strikes a good balance to me.

[LukaJCB]

👍 sounds good to me

[travisbrown]

Great, thanks all!

[gagandeepkalra]

Thank you @travisbrown for everything