Fix RPM build on COPR

[gschlager]

It looks like all builds in existing Fedora COPR repositories are either failing or out-of-date. I created a new one at https://copr.fedorainfracloud.org/coprs/gschlager/ruby/ which I intend to keep up-to-date.

But I had to make changes to rpm/ruby-install.spec and delete rpm/sources to make the build work.

[postmodern]

I'm pretty sure we need something like sources to check the integrity of the downloaded source archive. Will definitely accept the BuildRequires change.

[gschlager]

I'm not sure about the sources file. All I know is that the build failed because of it. It complained about the c24c12b179ce197e38b72ec2c80ef93e entry in the file.

[postmodern]

Going to close this because we should not remove the sources checksum verification file; unless it's being replaced with something better. In the future, I'd want to setup GitHub Actions to automatically build an RPM for new releases and push it to an official RPM repository.