Vagrantfile: Don't try to create extant link

`ln` fails if the link already exists. Since 7cbc9233 (Vagrantfile: Bail
out of provisioning if anything fails) this breaks re-provisioning a VM
with `vagrant reload --provision`.

Check if /home/vagrant/lisa already exists and if it does, leave it
alone.
diff --git a/Vagrantfile b/Vagrantfile
index eef54b7..98877c4 100644
--- a/Vagrantfile
+++ b/Vagrantfile
@@ -26,7 +26,9 @@
     sudo pip install ipython[notebook] pandas psutil wrapt
     sudo apt-get remove -y w3m
 
-    ln -s /vagrant /home/vagrant/lisa
+    if [ ! -e /home/vagrant/lisa ]; then
+       ln -s /vagrant /home/vagrant/lisa
+    fi
 
     cd /home/vagrant/lisa
     ANDROID_SDK_URL="https://dl.google.com/android/android-sdk_r24.4.1-linux.tgz"