Merge branch 'develop'

* develop:
  ALM-673 keep Gemfile.lock
  make installation of both pg and mysql gem the default
  change in brittle test
  ALM-679 small tweak in CSS
  ALM-679 only color title of events that are not citations
  ALM-679 color body of events that are not citations
  ALM-679 only color events that are not journal article
  ALM-679 color event
  ALM-679 different color for events
Search Twitter by DOI and URL. Closes #67.
Use number of active workers per source in rate-limit calculation. Closes #62.
updated documentation for ALM 3.2 release
added eLife ALM instance to documentation
include API version in cache key
give more space to source names in visualization
more space for source titles in visualizations
more space for source names
change placement of source charts
convert per_page parameter to integer
number of rows in API response should be between 1 and 50
small updates to documentation
Fixed release date in documentation
updated version string
ALM-625 started refactoring CrossRef import code. Added import model and query_url method
ALM-625 fixed small formatting issue
ALM-625 only import type journal-article
ALM-625 added get_data and parse_data methods to import
pretty json formatting