Fix release process

[tstirrat15]

Description

The last release workflow was broken. I took this as an opportunity to make this library behave like the others and set the version at release time.

Changes

• Move all dependency declarations into the gemspec, per recommendations here: https://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/
• Remove Gemfile.lock per recommendation ibid
• Add a ruby version file to reference with rvm
• Make workflow set version at release time

Testing

Review. See that the release goes.

[tstirrat15]

See comments