Fix two issues in the version cache code.

- The constructed git ref path was off (existing version.json files pretended a working system).
- Using atomicWriteJsonFile to avoid issues with multiple DUB instances running in parallel.
- If no .dub directory existed, the version.json file wasn't written.

source/dub/package_.d

@@ -606,7 +606,7 @@ private string determineVersionFromSCM(Path path)
 	if (exists(hpath)) {
 		auto head_ref = readText(hpath).strip();
 		if (head_ref.startsWith("ref: ")) {
-			auto rpath = (path ~ head_ref[5 .. $]).toNativeString();
+			auto rpath = (path ~ (".git/"~head_ref[5 .. $])).toNativeString();
 			if (exists(rpath))
 				head_commit = readText(rpath).strip();
 		}
@@ -627,7 +627,10 @@ private string determineVersionFromSCM(Path path)
 	auto ret = determineVersionWithGIT(path);
 
 	// update version cache file
-	writeJsonFile(vcachepath, Json(["commit": Json(head_commit), "version": Json(ret)]));
+	if (head_commit.length) {
+		if (!existsFile(path ~".dub")) createDirectory(path ~ ".dub");
+		atomicWriteJsonFile(vcachepath, Json(["commit": Json(head_commit), "version": Json(ret)]));
+	}
 
 	return ret;
 }