trigger build when pushed to the main branch

[jizusun]

This is really a trivial feature I have to admit, but I really love this project and hope to make it better ❤️

Since the default branch for newly-created repositories is now main on Github, we as a GitHub template would be better if we also follow the change:

• https://github.com/github/renaming
• https://github.blog/changelog/2020-10-01-the-default-branch-for-newly-created-repositories-is-now-main/

Once this pull request is merged, any maintain can rename our master branch to main very easily:

• https://github.com/github/renaming#renaming-existing-branches

[codecov]

Codecov Report

Merging #115 (4bb000b) into master (59d0665) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #115   +/-   ##
=======================================
  Coverage   50.00%   50.00%           
=======================================
  Files           1        1           
  Lines           2        2           
=======================================
  Hits            1        1           
  Misses          1        1           

Flag	Coverage Δ
Linux	50.00% <ø> (ø)
Windows	50.00% <ø> (ø)
macOS	50.00% <ø> (ø)

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[pellared]

I like this PR. Thanks for it and for your kind words as well.

Do you think that I should switch to main and remove master after this PR is merged?