remove node 0.10 sigterm code handling

The child process is no longer killed from the main process, instead it exits
upon receiving an 'exit' message. This makes the changes from
<https://github.com/sindresorhus/ava/pull/155> redundant.

lib/fork.js

@@ -79,7 +79,7 @@ module.exports = function (file, opts) {
 		});
 
 		ps.on('exit', function (code) {
-			if (code > 0 && code !== 143) {
+			if (code > 0) {
 				return reject(new AvaError(relFile + ' exited with a non-zero exit code: ' + code));
 			}