Introduce branching model

[tholewebgods]

• Have only one phantomjs.spec
• No version in the name
• Use (at least) tags (and branches) to track the version-specific changes
  • Tag: 1.5.0
  • Branch: 1.5

This should start with Phantom 1.5.0 (current in this Repo).

I would do this and pull-request it to you if you agree.