[QGIS-Developer] Disabled branch tests on Travis

[Nyall Dawson]

Hi list,

Given the absolute mess that Travis is right now (given docker's
limitations and Travis' upcoming price changes), I've taken the step
to disable the Travis runs on the master and release-3_16 branches.
This ONLY affects commits pushed direct to these branches, so I'm
hoping that by doing this we'll free up a free miniscule resource
scraps which might help alleviate some of the current pain with
getting pull requests tested by Travis.

(Travis is officially an old dog which needs taking out to the country
and a shotgun to the head. Bring on the glorious github actions
future!)

Nyall