latest ruby 2 build, rubygems 2 rc1
use the latest binary of ruby-head

Travis has changed to kill builds that don't output anything for five minutes.
In addition, Travis now uses rvm-head at all times, so we will now just use the
the latest binary build of ruby-head that's available by default, which is fine.
Modify file type

- 755 -> 644
Add gem for testing as a development dependency
Fix to use 'documentation' formatter

'specdoc' is not defined.
Add missing `require`

newgem spec should require 'spec_helper'.
Merge pull request #2254 from tricknotes/644

Modify file type
Merge pull request #2255 from tricknotes/fix-gem-template

Fix gem template
reference the source priority section

[ci skip]
Remove unnecessary constants

`Bundler::NULL` is already defined in 'lib/bundler.rb'.
Merge pull request #2258 from tricknotes/remove-unnecessary-constants

Remove unnecessary constants
Implement selective ignore of config with BUNDLE_IGNORE_CONFIG env var
Mention BUNDLE_IGNORE_CONFIG in the manpage
Add changelog entry for BUNDLE_IGNORE_CONFIG
it's all one conditional, I think