Add 1 to JSCS exit codes

In 0bb8c7e, `2` was added to the list of exit codes that JSCS
is expected to return, in response to #1006. However, it seems that JSCS
now also exits with code 1. In practice, this means that whenever you
save a file with a JavaScript syntax error, you see the following
message:

> syntastic: error: checker javascript/jscs returned abnormal status 1
> Press ENTER or type command to continue

After a quick look at the JSCS code, it appears that it is expected to
exit with codes 0, 1, or 2, so I am adding 1 to the list in this plugin.