Added pre-release suffix for development version of default gems

https://github.com/ruby/stringio/issues/81

ext/fiddle/lib/fiddle/version.rb

@@ -1,3 +1,3 @@
 module Fiddle
-  VERSION = "1.1.3"
+  VERSION = "1.1.3.dev"
 end

ext/stringio/stringio.c

@@ -13,7 +13,7 @@
 **********************************************************************/
 
 static const char *const
-STRINGIO_VERSION = "3.1.2";
+STRINGIO_VERSION = "3.1.2.dev";
 
 #include <stdbool.h>
 

ext/strscan/strscan.c

@@ -22,7 +22,7 @@ extern size_t onig_region_memsize(const struct re_registers *regs);
 
 #include <stdbool.h>
 
-#define STRSCAN_VERSION "3.1.1"
+#define STRSCAN_VERSION "3.1.1.dev"
 
 /* =======================================================================
                          Data Type Definitions