add fallback for Gem.loaded_specs

[ericmustin]

Gem.loaded_specs can sometimes be effected by issues similar to Capybara, where Ruby is not booted with rubygems enabled, but the gem can be require'd in and available. Therefore, as a fallback, it would be good to also check for the existence of the version number
defined by the library itself, instead of only accessing
it via Gem.loaded_specs.

It may be good to introduce this first to the api/sdk and then to the instrumentations to reduce the surface of the change

[ericmustin]

partially addressed by #1506

[ericmustin]

addressed by https://github.com/DataDog/dd-trace-rb/releases/tag/v0.50.0