Remove 'problems' key (#468)
Remove 'problems' key (#468)

The 'problems' key is deprecated in favour of 'exercises' so we can
remove the unused 'problems'
Insti/xruby,kotp/xruby,Insti/xruby,exercism/xruby,exercism/xruby,tommyschaefer/xruby,tommyschaefer/xruby,Insti/exercism-ruby,NeimadTL/ruby,Insti/exercism-ruby,NeimadTL/ruby,Insti/exercism-ruby,NeimadTL/ruby,kotp/xruby