Skip yanked crates when building world

So we can build world in a old fashion way in case of emergency
(Ref: #76)

Author: onur

src/docbuilder/crates.rs

@@ -36,6 +36,15 @@ fn crates_from_file<F>(path: &PathBuf, func: &mut F) -> Result<()>
                         .and_then(|n| n.as_string())
                         .ok_or("`vers` not found in JSON object"));
 
+        let yanked = try!(obj.get("yanked")
+                          .and_then(|n| n.as_boolean())
+                          .ok_or("`vers` not found in JSON object"));
+
+        // Skip yanked crates
+        if yanked {
+            continue;
+        }
+
         name.clear();
         name.push_str(crate_name);
         versions.push(format!("{}", vers));