Enhance specs for the triannon config

[dazza-codes]

For example, there are various specs on the configuration code in the triannon client:

• https://github.com/sul-dlss/triannon-client/blob/master/lib/triannon-client.rb#L20-L39
• https://github.com/sul-dlss/triannon-client/blob/master/lib/triannon-client/configuration.rb
• https://github.com/sul-dlss/triannon-client/blob/master/spec/lib/triannon-client/configuration_spec.rb

[dazza-codes]

This issue was motivated by a coveralls complaint, esp.

• https://coveralls.io/builds/3057626/source?filename=lib%2Ftriannon.rb#L30