RcppEnsmallen 0.2.11.1.1

• Upgraded to ensmallen 2.11.1: "The Poster Session Is Full" (2019-12-28)
  • Fix Lookahead Synchronization period type (#153).
  • Add Lookahead (#138).
  • Add AdaBound and AMSBound (#137).
  • SGD callback test 32-bit safety (big number) (#143).
  • Use "arbitrary" and "separable" terms in static function type checks (#145).
  • Remove 'using namespace std' from problems/ files (#147).
  • Add optional tests building. (#141).
  • Make code samples collapsible in the documentation. (#140).
• Switched deployment from TravisCI to GitHub Actions. (#17, #22)
• Removed check on header file inclusion (#21)