Security: disable the racket checker by default.

This checker executes the code in the files it checks. This is probably fine
if you wrote the files yourself, but it can be a problem if you're trying to
check third party files. If you are 100% willing to let Vim run the code in
your files, set g:enable_racket_racket_checker to 1 in your vimrc to enable
this checker: vim let g:enable_racket_racket_checker = 1 There is also a
buffer-local version of this variable, that takes precedence over it in the
buffers where it is defined.

Reference: #1773

plugin/syntastic.vim

@@ -19,7 +19,7 @@ if has('reltime')
     lockvar! g:_SYNTASTIC_START
 endif
 
-let g:_SYNTASTIC_VERSION = '3.7.0-138'
+let g:_SYNTASTIC_VERSION = '3.7.0-139'
 lockvar g:_SYNTASTIC_VERSION
 
 " Sanity checks {{{1

syntax_checkers/racket/racket.vim

@@ -42,7 +42,8 @@ endfunction
 
 call g:SyntasticRegistry.CreateAndRegisterChecker({
     \ 'filetype': 'racket',
-    \ 'name': 'racket'})
+    \ 'name': 'racket',
+    \ 'enable': 'enable_racket_racket_checker' })
 
 let &cpo = s:save_cpo
 unlet s:save_cpo