Use require_relative where possible.

adapter/main.rb

@@ -1,6 +1,6 @@
-require './encrypter'
-require './decrypter'
-require './string_io_adapter'
+require_relative 'encrypter'
+require_relative 'decrypter'
+require_relative 'string_io_adapter'
 
 reader = File.open('message.txt')
 writer = File.open('message.encrypted','w')

adapter/runtime_modifications/british_text_object.rb

@@ -1,4 +1,4 @@
-class BritishTextObject
+class BritishTextObject < TextObject
   attr_reader :string, :size_mm, :colour
 
   # ...

adapter/runtime_modifications/main.rb

@@ -1,6 +1,6 @@
-require './renderer'
-require './text_object'
-require './british_text_object'
+require_relative 'renderer'
+require_relative 'text_object'
+require_relative 'british_text_object'
 
 # Extend the BritishTexObject to include the methods
 # that the Renderer class requires