Build: switch from the "popular" browser set to "popular-qunit"

jQuery is moving toward future removal of support for IE6/7, Opera 12.1x
and Safari 5.1 whereas QUnit continues to support them. This commit switches
the used browser set to one containing all those browsers.

cc @jzaefferer

The definition of the browserSet is here: https://github.com/jquery/infrastructure/blob/1b4fe5b200467b268e4f7f3acc7d644cf7c3a577/modules/jquery/files/swarm/localSettings.json#L323-340