Remove Turn gem (incompatible with MiniTest > 5.0.0)
Rename test classes for consistency
Update to minitest 5
Add and configure guard for minitest
  Rename test_helper so that guard doesnt try to run it as a test
Add and configure minitap
Create very simple linked list to use in chapter two
Add #each to LinkedList
Change #size to make use of #each, and be slightly more useful as a
result
Complete exercises from chapter two