make bash script unset CDPATH to avoid no such file or directory errors
make bash script unset CDPATH to avoid no such file or directory errors

When the generated shell script mentioned at the end of `mix release` is
run with CDPATH set on a system where `/bin/sh` is really a `bash` then
`cd` can echo the directory it's changing to. This makes the setting of
`SCRIPT_DIR` fail:

```
 ==> Release successfully built!
    You can run it in one of the following ways:
      Interactive: rel/la_famiglia/bin/la_famiglia console
      Foreground: rel/la_famiglia/bin/la_famiglia foreground
      Daemon: rel/la_famiglia/bin/la_famiglia start
$ rel/la_famiglia/bin/la_famiglia console
rel/la_famiglia/bin/la_famiglia: line 5: cd: /Users/mike/Projects/influitive/la-famiglia/rel/la_famiglia/bin
/Users/mike/Projects/influitive/la-famiglia/rel/la_famiglia/bin/..: No such file or directory
```

bitwalker/distillery,bitwalker/distillery