#2132 - Replace all uses of foo_callback with foo

ale_linters/kotlin/ktlint.vim

@@ -4,7 +4,7 @@
 call ale#linter#Define('kotlin', {
 \   'name': 'ktlint',
 \   'executable': 'ktlint',
-\   'command_callback': 'ale#handlers#ktlint#GetCommand',
+\   'command': function('ale#handlers#ktlint#GetCommand'),
 \   'callback': 'ale#handlers#ktlint#Handle',
 \   'lint_file': 1
 \})