Update Travis badge to use master

[danielleadams]

No description provided.

[edmorley]

@danielleadams Hi! I followed a source link from another repo earlier and ended up looking at the master branch version of a file in this repo rather than main - which will progressively get more out of date over time.

Would it be possible to delete the master branch, so that GitHub's web UI redirects work and so existing links still point at the latest content?

This would break builds that hardcode master in the buildpack URL, however for this buildpack there are only 3 such apps - with last deploy dates of 2017-05, 2018-03 and 2019-02 respectively:
https://heroku.slack.com/archives/C1RU92Y1L/p1596575218489500?thread_ts=1596479766.483800&cid=C1RU92Y1L

...so this seems pretty safe to do.

[danielleadams]

Done 👍 where was the source link from?

[edmorley]

Thank you :-)

It was from heroku/buildpack-stdlib#43 (I'm reading up on bin/report for adding it to Python in the future)