Ensure #dump doesn't add a new line in config

Fixes #2821

* YAML#dump without any option parameters hard-wraps a string at 80
  characters and this causes errors while reading the config file
  and parsing it.

* There is another way to fix this: Modify the Regex that is used for
  parsing the config file. The current regex supports only a single line
  of "key: value" type statements. Presumably, YAML#load is not used for
  security reasons and so, this is the only other option.

	modified:   lib/bundler/settings.rb
	modified:   spec/commands/config_spec.rb
Use a new-line compatible regex for bundle config

* The option :line_width is available only in the YAML library and not
 in the older Syck and since we need to cater to installations that run
 Ruby versions below 1.9, this solution is not useful.

* The new regex will take into account newlines and will parse the
 configuration options accordingly. This makes it work even in Ruby
 1.8.7
Pass nil to net_http as proxy parameter not to use environment variables
Skip gems packaged with Ruby when packing.

This allows `bundle cache` and the like to work if you are using a gem that
is shipped with Ruby, like minitest 4.7.5.
Change else to elsif so that regular users just get the ENV variables.
Revert "Don't change local behavior with app cache"

It turns out this causes unwanted and unneeded network access.

Fixes #2882.

This reverts commit 81bf1d3f91689f4b376cc5d424646d5d0e4206c1.
Merge pull request #2878 from stlay/fix_no_proxy_request

Pass nil to net_http as proxy parameter not to use environment variables
Merge pull request #2869 from aughr/fetch_builtin_gems

Skip gems packaged with Ruby when packing.
Merge pull request #2823 from kgrz/fix-new-line-addition-in-config-file

Ensure #dump doesn't add a new line in config