Fix gem metadata when using aliases

[arthurnn]

Problem

After we upgraded our yaml parser to Psych latest, and start using safe_load, the YAML aliases on the metadata file stop working, they were been rejected by rubygems.org.

Solution

On safe_load we need to pass a true flag to allow aliases.

---

I added a regression test to make sure this doesnt happen again, aliases-0.0.0.gem is pretty much a test gem with one dependency like this (which includes a &id001 alias):

- !ruby/object:Gem::Dependency 
  name: bundler
  prerelease: false
  requirement: &id001 !ruby/object:Gem::Requirement 
    requirements: 
    - - ~>
      - !ruby/object:Gem::Version 
        version: "1.0"
  type: :development
  version_requirements: *id001

review @evanphx @qrush @dwradcliffe @sferik
cc @tenderlove