Run without deprecations, in BL 5.15

[jrochkind]

Previous commits/releases removed some BL 5.12+ deprecation warnings.

But one code path still called the deprecated solr_search_params.

Refactored to avoid that, using the new BL 5.15 SearchBuilder adjusting methods (so this now requires 5.15, gemspec updated). In the process, made some really messy code a bit cleaner too, hooray for new features.

Additional changes related to deprecations:

• Updated README with instructions for the optional parse basic query entry feature, that match current BL non-deprecated.
• Avoided some deprecation warnings from Rails itself by removing un-needed :action and :controller params from route helper calls.
• Rake testing setup uses new blacklight:index:seed instead of deprecated blacklight:solr:seed

[jrochkind]

Anyone want to code review? @jcoyne, @cbeer?

If I don't hear anything, and nobody tells me not to, then I may merge and release myself next week. (Feel free to tell me not to if you want me to wait longer for code review).