Prepare a 1.3.1.pre gem to test things for the 1.3.1 release.

Author: nirvdrum

docs-source/signpost.md

@@ -3,7 +3,7 @@
 Pick a `concurrent-ruby` version:
 
 * [master](./master/index.html)
-* [1.3.1 with edge 0.7.0](./1.2.3/index.html)
+* [1.3.1.pre with edge 0.7.0](./1.3.1.pre/index.html)
 * [1.1.10 with edge 0.6.0](./1.1.10/index.html)
 * [1.1.9 with edge 0.6.0](./1.1.9/index.html)
 * [1.1.8 with edge 0.6.0](./1.1.8/index.html)

lib/concurrent-ruby/concurrent/version.rb

@@ -1,3 +1,3 @@
 module Concurrent
-  VERSION = '1.3.1'
+  VERSION = '1.3.1.pre'
 end