Make CircleCI checkout merge commit

[fmassa]

This makes tests run not on the PR branch, but the PR branch rebased on top of master instead.
This is safer because the tests now signal the status of master if a PR gets merged.

[soumith]

if you tried it locally and you are getting the merge commit, go for it!

[codecov-io]

Codecov Report

Merging #1344 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1344   +/-   ##
=======================================
  Coverage   65.59%   65.59%           
=======================================
  Files          75       75           
  Lines        5819     5819           
  Branches      892      892           
=======================================
  Hits         3817     3817           
  Misses       1735     1735           
  Partials      267      267

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