Remove non-developer dependencies.

[vxern]

For it to be a legitimate claim that the package is 'lightweight', the package should have as few dependencies as possible.

• Remove sprint.
  This will be a breaking change, as the library will have to throw an exception instead.
• Remove web_scraper.
  Instead of scraping webpages itself to obtain the contents of the robots.txt file, the package could instead accept a content parameter passed into it by the developer using the library, thus making the package both more flexible and legitimately 'lightweight'. This will also be a breaking change.

[vxern]

Removed in #5.