Update the buildpack to conform to the latest Heroku API

Heroku has deprecated the config_vars key in .release a long time
ago and recommends settings config vars using a shell script in
`.profile.d`.

This patch updates `bin/compile` to create the recommended file and
removes the config_vars key from the `.release` file.

bin/compile

@@ -106,6 +106,12 @@ else
   cp -r $CACHE_DIR/bin/* $BUILD_DIR/nginx/
 fi
 
+# Update the PATH
+mkdir -p $BUILD_DIR/.profile.d
+cat > $BUILD_DIR/.profile.d/nginx.sh <<"EOF"
+export PATH="$PATH:$HOME/nginx"
+EOF
+
 cd $CUR_DIR
 
 # build nginx config unless overridden by user