serrano v0.3.6

• Updated dependency versions
• Now using vcr gem for caching HTTP requests for test suite (#38)
• Changed base URL for Crossref API for all requests to https (#37)
• Updated docs for facet parameter that it can be either a
  boolean or a query string, e.g,. license:* to facet by license (#36)
• Documented in offset parameter the max of 10K instituted somewhat
  recently (#34)
• Documented in top level Serrano module and in README the new
  rate limit on all requests - which varies through time. (#32)