Apply for travis-ci.com OSS credits

[dennisameling]

As mentioned in #349 (comment), travis-ci.org is shutting down soon. The new travis-ci.com doesn't offer free builds by default, but rather a one-time free credit allocation (10,000 credits). You can apply for OSS credits though:

Would you mind emailing them at support [at] travis-ci [dot] com to apply for these credits? Even if we merge #334, you'll still need Travis for arm64 builds (or run QEMU on GitHub Actions which is incredibly slow).