Allow vendoring of gems

[Eileen M. Uchitelle]

Bundler supports vendoring gems into a specified directory. Some apps rely on this feature so they don't need to download from RubyGems if the gem is already present.

Once vendoring is implemented we'll want to have a way to clean the gems that are no longer used. To make that work for apps that dual-boot in multiple Rails version we'll need a way to set a list of gems to not clean.

[Adam Hess]

It looks like you can specify a directory to install gems into with an environment variable today:

$ ls                                                                                                                                                                                                                              08:15:40
Gemfile      Gemfile.lock test.rb

$ env GEL_STORE=./gems gel exec ruby test.rb
Traceback (most recent call last):
	2: from test.rb:1:in `<main>'
	1: from /usr/local/Cellar/gel/0.3.0/lib/gel/compatibility/rubygems.rb:197:in `require'
/usr/local/Cellar/gel/0.3.0/lib/gel/compatibility/rubygems.rb:197:in `require': cannot load such file -- httparty (LoadError)

$ env GEL_STORE=./gems gel install
Installing multi_xml (0.6.0)                                
Installing mime-types (3.3.1)                     
Installing mime-types-data (3.2019.1009)
Installing httparty (0.18.0)
Installed 4 gems

$ env GEL_STORE=./gems ../gel/exe/gel exec ruby test.rb
hi

$ ls
Gemfile      Gemfile.lock gems         test.rb

[Matthew Draper]

Gel still doesn't support installing into a specific directory, because of the custom database it needs to track which files are available. But it does now support reading locally-bundled .gem files from vendor/cache/*.gem in place of server-supplied equivalents, which seems like a fair compromise: the local Gel instance is still able to maintain book-keeping of installed files, while the managed app directory is able to pre-package dependencies to avoid a live server requirement.