(#29) Add missing release notes to generated packages

[AdmiringWorm]

Description Of Changes

This adds a version substitution to the nuspec file to automatically
insert the version to the release notes link when a package is
generated.

Motivation and Context

We want to have a link to the release notes when we push a package to the Chocolatey Community Repository.

Testing

1. Run .\build.ps1.
2. Navigate to code_drop\Packages\Chocolatey.
3. Extract the nupkg file.
4. Open up the nuspec file.
5. Ensure that the release notes element have the version number specified in the URL.

Operating Systems Testing

• Windows 10

Change Types Made

• Bug fix (non-breaking change).
• Feature / Enhancement (non-breaking change).
• Breaking change (fix or feature that could cause existing functionality to change).
• Documentation changes.
• PowerShell code changes.

Change Checklist

• Requires a change to the documentation.
• Documentation has been updated.
• Tests to cover my changes, have been added.
• All new and existing tests passed?
• PowerShell code changes: PowerShell v2 compatibility checked?

Related Issue

Fixes #29

[corbob]

LGTM!