Fix nightly builds

[fmassa]

Nightly builds on master have been failing after #1344 got merged with error

#!/bin/bash -eo pipefail
set -ex
git fetch --force origin ${CIRCLE_BRANCH}/merge:merged/${CIRCLE_BRANCH}
git checkout "merged/$CIRCLE_BRANCH"
+ git fetch --force origin master/merge:merged/master
fatal: Couldn't find remote ref master/merge
Killed by signal 1.

Exited with code 128

because the PR assumed that CI would only run on PR branches.
This PR skips rebasing if CI is run on master.

[fmassa]

Test failures are unrelated. It seems that Travis CI has less CPU memory than CircleCI (for the machines we are using). Those failing tests should be disabled on Travis

[fmassa]

cc @stephenyan1231 for the failures on video_reader. It looks like the tests are using too much CPU memory for Travis.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@0982395). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #1374   +/-   ##
=========================================
  Coverage          ?   63.83%           
=========================================
  Files             ?       78           
  Lines             ?     6147           
  Branches          ?      940           
=========================================
  Hits              ?     3924           
  Misses            ?     1945           
  Partials          ?      278

Impacted Files	Coverage Δ
torchvision/datasets/utils.py	62.11% <ø> (ø)

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