Removed syntastic and replaced it with ale

Read more here:
https://github.com/w0rp/ale

sources_non_forked/ale/ale_linters/sml/smlnj.vim

@@ -0,0 +1,9 @@
+" Author: Paulo Alem <paulo.alem@gmail.com>, Jake Zimmerman <jake@zimmerman.io>
+" Description: Single-file SML checking with SML/NJ compiler
+
+call ale#linter#Define('sml', {
+\   'name': 'smlnj',
+\   'executable_callback': 'ale#handlers#sml#GetExecutableSmlnjFile',
+\   'command': 'sml',
+\   'callback': 'ale#handlers#sml#Handle',
+\})