Obtain gemspecs from the local system cache if available.

Ref #2444

`bundle install` (when the Gemfile is modified) and `bundle update` commands take a long time when latency is high between the system running the command and Rubygems.

This PR works around part of this problem by obtaining gemspecs from the local gem specification directories if they're available.

I need assistance writing a spec for this PR, I can't quite work out which combination of builders I need to use to fake out an already existing specification file. Am I right in assuming a test for this should go in `spec/install/gems/simple_case_spec.rb`?

## Benchmarks

of an `bundle update` on an already up-to-date `Gemfile.lock`:

### With local specificiations patch
`bundle _1.3.5.mp_ update  20.10s user 0.62s system 29% cpu 1:10.70 total`

### Original flavour
`bundle update  20.15s user 0.72s system 10% cpu 3:20.49 total`