Allow multiple - in a version string

Right now we were assuming that there would be no `-` in a version.
That was breaking things.

This allows more flexibility for versions like:

1.0.0-beta.2

Ref facebook#455
Fixes facebook#450

lib/server/versionFallback.js

@@ -120,7 +120,13 @@ files.forEach(file => {
   if (!(metadata.original_id in available)) {
     available[metadata.original_id] = new Set();
   }
-  const version = metadata.id.split('-')[1];
+  // The version will be between "version-" and "-<docid>"
+  // e.g. version-1.0.0-beta.2-doc1 => 1.0.0-beta.2
+  // e.g. version-1.0.0-doc2 => 1.0.0
+  const version = metadata.id.substring(
+    metadata.id.indexOf('-') + 1,
+    metadata.id.lastIndexOf('-')
+  );
   available[metadata.original_id].add(version);
 
   if (!(version in versionFiles)) {