Gist ID should be a hex number, not just an int

Github changed from using numeric IDs for gists to using hex numbers.
This fixes the gem to handle that.

Also updated spec: gists should be hex nums (and fail on anything else)
Merge pull request #12 from jm3/develop

Gist ID should be a hex number, not just an int
Update the URL of the blog
Update CHANGELOG.md
Bump version
Update dependencies
Fix rspec deprecation warnings