systemd on 15.04 won't log stdout of unit

I'm currently trying to make a systemd unit as a web server. Currently, my foo.service file looks as follows:

[Unit]
Description=The Foo Web Server

[Service]
Type=simple
ExecStart=/opt/foo/.cabal-sandbox/bin/foo

[Install]
WantedBy=multi-user.target

The foo executable automatically logs all HTTP requests to stdout - this is well tested. However, when I view the logs with journalctl -u foo, I only get output like this:

...
May 06 17:46:57 localhost systemd[1]: Stopping The Foo Web Server...
May 06 17:46:57 localhost systemd[1]: Started Foo Web Server.
May 06 17:46:57 localhost systemd[1]: Starting The Foo Web Server...
May 06 17:47:08 localhost systemd[1]: Stopping The Foo Web Server...
May 06 17:47:08 localhost systemd[1]: Started The Foo Web Server.
May 06 17:47:08 localhost systemd[1]: Starting The Foo Web Server...

Could someone explain why it's not logging all stdout output? I looked briefly at this previous question, but it doesn't help - however it alluded to something along the lines of "...may not work for systems that don't use full systemd" - would this be the case for Ubuntu 15.04? Thank you in advance, any help with this would be much appreciated!

In fact, buffering in UNIX depends on the context: when stdout is redirected to something interactive like a console - it is usually line-buffered, otherwise it is fully buffered.

Buffering may be changed inside the application using setvbuf library call.

But it can also be done with stdbuf command on launch:

ExecStart=/usr/bin/stdbuf -oL /opt/foo/.cabal-sandbox/bin/foo

(for line-buffered case)