Backport to 186

This pull request add compatibility with ruby 1.8.6 by requiring backports and adding Time#nsec.

I wasn't sure about how to configure Travis, so I left that file untouched. 

I tested this on a real project running 1.8.6-p383 and it successfully bundle install gems from both rubygems and github. 

I had some issue running the test suite:
```
➜  bundler git:(backport_to_186) ✗ rake spec
/Users/Nerian/projects/bundler/lib/bundler/lockfile_parser.rb:56:in `<class:LockfileParser>': uninitialized constant Bundler::Source::SVN (NameError)
	from /Users/Nerian/projects/bundler/lib/bundler/lockfile_parser.rb:14:in `<module:Bundler>'
	from /Users/Nerian/projects/bundler/lib/bundler/lockfile_parser.rb:13:in `<top (required)>'
	from /Users/Nerian/.rvm/gems/ruby-2.1.1/gems/bundler-1.6.2/lib/bundler/definition.rb:58:in `initialize'
	from /Users/Nerian/.rvm/gems/ruby-2.1.1/gems/bundler-1.6.2/lib/bundler/dsl.rb:178:in `new'
	from /Users/Nerian/.rvm/gems/ruby-2.1.1/gems/bundler-1.6.2/lib/bundler/dsl.rb:178:in `to_definition'
	from /Users/Nerian/.rvm/gems/ruby-2.1.1/gems/bundler-1.6.2/lib/bundler/dsl.rb:11:in `evaluate'
	from /Users/Nerian/.rvm/gems/ruby-2.1.1/gems/bundler-1.6.2/lib/bundler/definition.rb:26:in `build'
	from /Users/Nerian/.rvm/gems/ruby-2.1.1/gems/bundler-1.6.2/lib/bundler.rb:153:in `definition'
	from /Users/Nerian/.rvm/gems/ruby-2.1.1/gems/bundler-1.6.2/lib/bundler.rb:136:in `load'
	from /Users/Nerian/.rvm/gems/ruby-2.1.1@global/gems/rubygems-bundler-1.4.3/lib/rubygems-bundler/noexec.rb:46:in `candidate?'
	from /Users/Nerian/.rvm/gems/ruby-2.1.1@global/gems/rubygems-bundler-1.4.3/lib/rubygems-bundler/noexec.rb:92:in `setup'
	from /Users/Nerian/.rvm/gems/ruby-2.1.1@global/gems/rubygems-bundler-1.4.3/lib/rubygems-bundler/noexec.rb:124:in `check'
	from /Users/Nerian/.rvm/gems/ruby-2.1.1@global/gems/rubygems-bundler-1.4.3/lib/rubygems-bundler/noexec.rb:131:in `<top (required)>'
	from /Users/Nerian/.rvm/rubies/ruby-2.1.1/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:135:in `require'
	from /Users/Nerian/.rvm/rubies/ruby-2.1.1/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:135:in `rescue in require'
	from /Users/Nerian/.rvm/rubies/ruby-2.1.1/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:144:in `require'
	from /Users/Nerian/.rvm/gems/ruby-2.1.1@global/gems/rubygems-bundler-1.4.3/lib/rubygems_executable_plugin.rb:4:in `block in <top (required)>'
	from /Users/Nerian/.rvm/gems/ruby-2.1.1@global/gems/executable-hooks-1.3.1/lib/executable-hooks/hooks.rb:50:in `call'
	from /Users/Nerian/.rvm/gems/ruby-2.1.1@global/gems/executable-hooks-1.3.1/lib/executable-hooks/hooks.rb:50:in `block in run'
	from /Users/Nerian/.rvm/gems/ruby-2.1.1@global/gems/executable-hooks-1.3.1/lib/executable-hooks/hooks.rb:49:in `each'
	from /Users/Nerian/.rvm/gems/ruby-2.1.1@global/gems/executable-hooks-1.3.1/lib/executable-hooks/hooks.rb:49:in `run'
	from /Users/Nerian/.rvm/gems/ruby-2.1.1/bin/ruby_executable_hooks:10:in `<main>'
```