Use rails-version-specific sqlite3 gem versions

[mjankowski]

As mentioned here - #217 (comment) - there are some CI and local failures in the repo now b/c the newer rails-edge appraisal run expects sqlite3 gem version 2.x, but we have 1.7 specified in the Genfile (and thus inherited by the appraisal gemfiles). Changes here:

• Relax the main Gemfile to not include a version, which will default to the later versions, which presumably makes sense for anyone doing local non-appraisal-run development?
• Update the appraisals to lock to 1.7 for rails-7 path and 2.x for the rails-edge path

For me locally this gets the previously failing appraisal test run to pass w/out the sqlite3 version conflict error ... assume same will occur on CI.