fix raising error when path contains unexisting users home dir

When I put some gem to `gemfile` like

    gem 'gemname', path: "~someunexistinguser/path/to/gemname"

I get a message

```
Unfortunately, a fatal error has occurred. Please see the Bundler troubleshooting documentation at http://bit.ly/bundler-issues. Thanks!
/home/vagrant/.rvm/gems/ruby-2.0.0-p247@global/gems/bundler-1.3.5/lib/bundler/source/path.rb:62:in `expand_path': user someunexistinguser doesn't exist (ArgumentError)
	from /home/vagrant/.rvm/gems/ruby-2.0.0-p247@global/gems/bundler-1.3.5/lib/bundler/source/path.rb:62:in `expand_path'
	from /home/vagrant/.rvm/gems/ruby-2.0.0-p247@global/gems/bundler-1.3.5/lib/bundler/source/path.rb:62:in `eql?'
	from /home/vagrant/.rvm/gems/ruby-2.0.0-p247@global/gems/bundler-1.3.5/lib/bundler/definition.rb:466:in `block (2 levels) in converge_dependencies'
    ...
```

So, I propose a solution and I think it's not the best. I'm waiting for feedback