Change JSCS exit codes from [0, 2] to [0, 1]

In 0bb8c7e5f627d3c87b, `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 exits only with codes 0 and 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 only expected
to exit with codes 0 or 1, so I am changing the list in this plugin.