Skip clone gem when passed 'GEMSRC_SKIP'

[unasuke]

I use bundle install --force command often for re-install several gems.
It takes much time by non cloneable url in gemspec.
Or, sometimes want to skip clone explicitly.

in my case

• Gemfile
• Gemfile.lock

bundle install --force

% time bundle install --force
Installing rake 11.2.2
    exists /Users/unasuke/src/github.com/ruby/rake
(snip)...
Bundle complete! 28 Gemfile dependencies, 97 gems now installed.
Use `bundle show [gemname]` to see where a bundled gem is installed.
bundle install --force  48.37s user 26.60s system 24% cpu 5:11.44 total

mean of 5 times : 5:07.43

GEMSRC_SKIP=true bundle install --force

% time GEMSRC_SKIP=true bundle install --force
Installing rake 11.2.2
Installing concurrent-ruby 1.0.2
(snip)...
Bundle complete! 28 Gemfile dependencies, 97 gems now installed.
Use `bundle show [gemname]` to see where a bundled gem is installed.
GEMSRC_SKIP=true bundle install --force  42.75s user 20.86s system 99% cpu 1:03.72 total

mean of 5 times : 1:04.55

[amatsuda]

@unasuke Thank you, I like this!