Remove the dependency on Rails

[aub]

The main goal of this is to remove the dependency on the rails gem, since the library seems to only use activesupport. Along the way I found that some of the dependencies were specified in a wonky way and that the tests didn't work at all with Appraisal, so this also fixes those issues. The changes were:

• Specify versions of pg in the Appraisals file because the tests would crash on dependencies being incorrect running against 1.0.
• Specify a version of minitest for testing rails 5 because the default included version had a bug that crashed the specs.
• Remove development dependencies from the Gemfile and move them to the gemspec where they ought to go.
• Remove rails and pg as dependencies since they're only needed for the tests and there specified with Appraisal. Add activesupport as a dependency.

[ramontayag]

This should work but I believe it will continue to require active record as well.

[ramontayag]

@aub I've been interested in seeing this happen. I believe we'd need some way to easily manage the migrations in the tests, still rely on ActiveRecord, but at least we can remove the dependency on Rails directly.

[ramontayag]

I realized that we don't have to remove the dependency for development. Just production! Check this out: https://github.com/G5/storext/pull/54