Adjust tests for Travis CI build.

Adjusted `t/sdk/proxy.t.js` for Linux. On Darwin the server process
exits successful, while on Linux the exit is null and the signal is
reported.

Updated the `stdout` listener to fire only once. It was firing twice on
Travis CI, causing the test to setup a second proxy which failed because
the ports were already bound.

t/sdk/proxy.t.js

@@ -6,7 +6,7 @@ require('./proof')(3, function (step, say, ok, equal, execute, proxy) {
   var other = step();
   server.on('error', function (error) { throw error });
   server.stderr.pipe(process.stderr);
-  server.stdout.on('data', function (chunk) {
+  server.stdout.once('data', function (chunk) {
     proxy(9293, 9593, 9000, function (buffer) { return buffer });
     client = execute('integer/client', [ '127.0.0.1', 9293 ]);
     client.stderr.pipe(process.stderr);
@@ -18,7 +18,8 @@ require('./proof')(3, function (step, say, ok, equal, execute, proxy) {
     });
   });
   server.on('close', function (code, signal) {
-    equal(code, 0, 'server closed');
+    if (code != null) equal(code, 0, 'server closed');
+    else equal(signal, 'SIGTERM', 'server closed');
     callback(null);
   });
 });