changing default branch name from master to main over TTWG repositories

   hi all,

# feel free to redistribute, if you need.

   Taking long time from discussion on call [1], followings are summaries of current condition
and plan to propose.
*1 https://www.w3.org/2020/11/19-tt-minutes.html#t05

   Attached list is developed based on data from validate-repos [2] via quick listing tool [3].

*2 https://w3c.github.io/validate-repos/
*3 https://himorin.github.io/w3c-memo/github/listrepo.html (put 34314 as group id to show a list)

   GitHub now provides a page to change default branch with changing branch name of the
target, repointing PRs to new default branch, updating source of gh-pages, updating
branch protection rules, etc.
   So, let me propose

- to change the default branch name from 'master' to 'main' for most of repositories (see list below; except 3)
- update configuration of TravisCI for ttml1/ttml2/ttml3 repositories

   After this, you will be required to update your local branch name from 'master' to
'main' like via:
git branch -m master main


----
   List of repositories:

   IMSC-1.0.1_Text_TestContent
   IMSC-1.1_Image_TestContent
   IMSC-1.1_Text_TestContent
   tt-module-cjk-ext-1
   tt-module-css
   tt-module-registry
   tt-module-vr
   ttml1-tests
   ttml2-tests
   ttml3-tests
   adpt
   imsc
   imsc-tests
   imsc-vnext-reqs
   ping-hdr-pg
   tt-module-karaoke
   tt-module-live
   tt-reqs
   ttwg
   ttml1
   ttml2
   ttml3
   timed-text-dashboard (is still used now??)
   ttml-webvtt-mapping