Modify and add unit tests for the purl method in LinkedData::Client::Models::Class

[jvendetti]

• Modifies the purl method to check class IDs for a PURL host, rather than the string 'purl' (see this comment).
• Makes the PURL host configurable, and sets a default value
• Adds several unit tests for the purl method
• Configures PURL host and prefix values in config.test.rb for the purposes of unit text execution
• Adds a development dependency on faraday-follow_redirects for testing proper PURL resolution
• Moves the gem version specifier to a module
• Fixes some RuboCop warnings