adds in the question marks for consistency

[afbreilyn]

fixes #35

[yordis]

@afbresley @alexdovzhanyn I think you should be using ? when it gives you a boolean back, is_ when you should be checking some branches like you are doing right now.

[yordis]

@afbresley @alexdovzhanyn from Credo

 [R] ↗ Predicate function names should not start with 'is', and should end in a
┃       question mark.
┃       lib/ledger.ex:44 #(UltraDark.Ledger.is_empty?)

[alexdovzhanyn]

@afbresley @yordis fair enough, credo pretty explicitly sets the standard