Use a generic user-agent for HTTP requests
Add an official provider for Spotify
Don't test against ruby 1.8.7

Nokogiri requires ruby >= 1.9.3 so there's no point in testing 1.8.7.
Test against modern MRI rubies in CI
Merge pull request #37 from venuu/modernize-travis

Upgrade CI build matrix
Use a library-specific user agent
Update CHANGELOG
Add informal documentation of Spotify's OEmbed API
Merge pull request #36 from venuu/spotify-provider

Add a provider for Spotify
Update CHANGELOG for Pull #37
Bump to 0.8.11