Fixes 'nil can't be coerced into Fixnum' error.

    Chord.transpose(['Cm','F','Am','G']).to('Bb')

TypeError:
       nil can't be coerced into Fixnum
     # ./lib/transpose_chords.rb:17:in `-'
     # ./lib/transpose_chords.rb:17:in `to'
     # ./spec/lib/transpose_chords_spec.rb:19:in `block (3 levels) in
<module:TransposeChords>'
Transposes a progression with inverted chords.

Chord.transpose(['C/E','F/A','Am/G','G/B']).to('D').should ==
['D/F#','G/B','Bm/A','A/C#']