As was written above, probably, you have not enough RAM.
I solved problem by adding SWAP file on my Ubuntu 14.04 server:
Under the root
:
dd if=/dev/zero of=/swapfile bs=1024 count=512k
mkswap /swapfile
swapon /swapfile
Add next line to /etc/fstab
:
/swapfile none swap sw 0 0
and:
echo 0 > /proc/sys/vm/swappiness
sudo chown root:root /swapfile
sudo chmod 0600 /swapfile
check SWAP(maybe need reloading):
swapon -s