Don't depend on non-existing Exercise
Don't depend on non-existing Exercise

These exercises was on the Go track, but isn't present in the Ruby track:

* bank-account
* error-handling
* parallel-letter-frequency
* tree-building

Insti/exercism-ruby,Insti/xruby,exercism/xruby,Insti/exercism-ruby,exercism/xruby,NeimadTL/ruby,NeimadTL/ruby,NeimadTL/ruby,Insti/xruby,Insti/exercism-ruby