Use File.exist? instead of File.exists?

The latter is deprecated.
Ref: https://ruby-doc.org/core-2.4.0/File.html#method-c-exists-3F

And the former is exist since at least Ruby 2.2 (it is the oldest version that supported
by sassc-rails).
Ref: https://ruby-doc.org/core-2.2.0/File.html#method-c-exist-3F

Author: tricknotes

lib/sassc/rails/importer.rb

@@ -108,7 +108,7 @@ def imports(path, parent_path)
 
             EXTENSIONS.each do |extension|
               try_path = File.join(search_path, file_name + extension.postfix)
-              if File.exists?(try_path)
+              if File.exist?(try_path)
                 record_import_as_dependency try_path
                 return extension.import_for(try_path, parent_dir, options)
               end

test/sassc_rails_test.rb

@@ -248,9 +248,9 @@ def test_allows_for_inclusion_of_inline_source_maps
   #    css_output = asset_output('css_application.css')
   #    assert_match /globbed/, css_output
 
-  #    if File.exists?("#{app_root}/log/development.log")
+  #    if File.exist?("#{app_root}/log/development.log")
   #      log_file = "#{app_root}/log/development.log"
-  #    elsif File.exists?("#{app_root}/log/test.log")
+  #    elsif File.exist?("#{app_root}/log/test.log")
   #      log_file = "#{app_root}/log/test.log"
   #    else
   #      flunk "log file was not created"