Remove require_relative (#117)

Require relative is only available in modern rubies. Let's keep this
working on some of the older versions as well.

https://github.com/neo/ruby_koans/pull/117

src/about_triangle_project.rb

@@ -1,7 +1,7 @@
 require File.expand_path(File.dirname(__FILE__) + '/neo')
 
 # You need to write the triangle method in the file 'triangle.rb'
-require_relative 'triangle.rb'
+require './triangle'
 
 class AboutTriangleProject < Neo::Koan
   def test_equilateral_triangles_have_equal_sides
@@ -22,4 +22,3 @@ class AboutTriangleProject < Neo::Koan
     assert_equal :scalene, triangle(5, 4, 2)
   end
 end
-