BUG-149: Fix compatibilities performance bug

Use fetcher instead of new when we use Gem::SpecFetcher

Fixes: #149

Author: fbuys

lib/next_rails/gem_info.rb

@@ -109,7 +109,7 @@ def unsatisfied_rails_dependencies(rails_version: nil)
 
     def find_latest_compatible(rails_version: nil)
       dependency = Gem::Dependency.new(@name)
-      fetcher = Gem::SpecFetcher.new
+      fetcher = Gem::SpecFetcher.fetcher # Use fetcher instead of ::new to reduce object allocation.
 
       # list all available data for released gems
       list, errors = fetcher.available_specs(:released)

spec/next_rails/gem_info_spec.rb

@@ -83,7 +83,7 @@
       # Set up a mock SpecFetcher to return an empty list
       fetcher_double = double("spec_fetcher")
       allow(fetcher_double).to receive(:available_specs).and_return([[],[]])
-      allow(Gem::SpecFetcher).to receive(:new).and_return(fetcher_double)
+      allow(Gem::SpecFetcher).to receive(:fetcher).and_return(fetcher_double)
 
       gem_info = NextRails::GemInfo.new(gem)
       gem_info.find_latest_compatible