clean up build website script

tools/build-website.sh

@@ -1,9 +1,16 @@
 #!/bin/sh
 set -e
 
-git clone git@github.com:babel/babel.github.io.git
-cd babel.github.io
-ln -s .. _babel
+if [ ! -d ./_babel.github.io ]; then
+  git clone git@github.com:babel/babel.github.io.git _babel.github.io
+fi
+
+cd _babel.github.io
+
+if [ ! -d ./_babel ]; then
+  ln -s .. _babel
+fi
+
 make build
 git commit -am "${TRAVIS_TAG}"
 git push "https://${GH_TOKEN}@github.com/babel/babel.github.io"