Fix some compatibility issues

[mylanconnolly]

Hello,

I was interested in trying this project out to bundle my Rails application into a deb and had to do a few tweaks to make this work:

1. Add method EnvValue#strip to return an EnvValue instead of a String (this might be a Ruby 3.0 change?)
2. I noticed the default branch of the Ruby buildpack changed from master to main so I changed the default value to make the tests pass.

After testing, I was able to use it to successfully build a deb of my project.

Since I was testing this out in GitHub Actions, I left my workflow file for now. If you'd rather not use it, I can remove it. Right now it is testing against the following:

• Ruby versions 1.9, 2.1, 2.2, 2.3, 2.4, 2.6, 2.7, 3.0
• Ubuntu versions 18.04 and 20.04

I was aiming to match the versions that were covered in Travis CI with exception of Ubuntu 16.04 which I don't think is available for GitHub Actions.

Hopefully this looks good, if you have any concerns please let me know.

Thanks for the project!