Undocumented option "bash_hack" is gone.
New function syntastic#util#system(), that sets up the environment and
is now used consistently everywhere.
Unrelated fix for the elixir checker (syntax).
On Windows R is run with CWD set to the directory used at install time,
rather than the current directory. The official workaround for this is
to add links to R from the home directories of each of your projects.
We can't do that in Vim, but we can call `setwd()` to Vim's idea of
current directory. This mimics the behaviour of R on UNIX.
