Switch to external depgraph.gemspec so DepGraph can be used with Bundler

This PR externalizes the gemspec within the Rakefile to its own `depgraph.gemspec` file. Inside the Rakefile we now `require 'bundler/gem_tasks'` which now provides the rake tasks for packaging and release, e.g.:
```
D:\Repos\_github\depgraph>rake -T
rake build    # Build depgraph-0.11.0.gem into the pkg directory
rake install  # Build and install depgraph-0.11.0.gem into system gems
rake release  # Create tag v0.11.0 and build and push depgraph-0.11.0.gem t...
rake spec     # Run RSpec code examples
```

Having the `depgraph.gemspec` it its own file allows you to define depgraph as dependency in a `Gemfile` so that it works with Bundler, e.g.:
```
source 'https://rubygems.org'

# use steve's version of depgraph with -type cookbook support
gem 'depgraph', git: 'https://github.com/leftathome/depgraph.git'
```