Fix #1168 - Make the ruby linter executable configurable

ale_linters/ruby/ruby.vim

@@ -1,10 +1,22 @@
 " Author: Brandon Roehl - https://github.com/BrandonRoehl
 " Description: Ruby MRI for Ruby files
 
+call ale#Set('ruby_ruby_executable', 'ruby')
+
+function! ale_linters#ruby#ruby#GetExecutable(buffer) abort
+    return ale#Var(a:buffer, 'ruby_ruby_executable')
+endfunction
+
+function! ale_linters#ruby#ruby#GetCommand(buffer) abort
+    let l:executable = ale_linters#ruby#ruby#GetExecutable(a:buffer)
+
+    return ale#Escape(l:executable) . ' -w -c -T1 %t'
+endfunction
+
 call ale#linter#Define('ruby', {
 \   'name': 'ruby',
-\   'executable': 'ruby',
+\   'executable_callback': 'ale_linters#ruby#ruby#GetExecutable',
+\   'command_callback': 'ale_linters#ruby#ruby#GetCommand',
 \   'output_stream': 'stderr',
-\   'command': 'ruby -w -c -T1 %t',
 \   'callback': 'ale#handlers#ruby#HandleSyntaxErrors',
 \})