(FACT-186) Consolidate gem runtime dependencies

Previously, the Gemfile and ext/project_data.yaml both described gem runtime
dependencies, the former requiring an exact match, the latter a pessimistic
match.

This commit makes project_data.yaml be the single source of truth, and it
specifyies exact versions.

Rubygems and bundler have different ideas about what constitutes a platform.
Rubygems requires an arch and os, e.g. x86_mingw32, while bundler has a
hard-coded list, see Bundler::Dependency::PLATFORM_MAP. As result, we need
to use the rubygems platform when building the facter gem, but use the
bundler platform when expressing gem dependencies.