Fix 'rake test:unit' for Windows.

Windows uses ";" instead of ":" for path separation.
We don't need to care about this difference, when using two "-I"

This fixes commit f15b942 .

Rakefile

@@ -4,7 +4,7 @@ require 'bundler/gem_tasks'
 namespace :test do
   desc "Test MiniPortile by running unit tests"
   task :unit do
-    sh "ruby -w -W2 -I.:lib -e \"#{Dir["test/test_*.rb"].map{|f| "require '#{f}';"}.join}\" -- -v"
+    sh "ruby -w -W2 -I. -Ilib -e \"#{Dir["test/test_*.rb"].map{|f| "require '#{f}';"}.join}\" -- -v"
   end
 
   desc "Test MiniPortile by compiling examples"