README.md: Update build status badges
README.md: Update build status badges

As travis-ci gets not triggered anymore, continuous integration has
been changed in commit 57a5a7e0 to use GitHub Actions. This change
updates the build status badges to report the GitHub Actions result
for the branches master and develop.

authmillenon/libcoap,authmillenon/libcoap,authmillenon/libcoap