Remove root directory from library search path for tests

It isn't in the gemspec's require_path, so we shouldn't add any dependence
on it.

Rakefile

@@ -9,7 +9,7 @@ task(default: [:test, :rubocop])
 
 desc('run test suite with default parser')
 Rake::TestTask.new(:base_test) do |t|
-  t.libs << '.' << 'lib' << 'test'
+  t.libs << 'lib' << 'test'
   t.test_files = FileList['test/{integration,unit}/**/*_test.rb']
   t.verbose    = false
 end