Use a glob to find the location of libsodium's shared library

Apparently in some build environments (e.g. Windows) the "symlinked"
version is not always created. This should (hopefully) always find it.

lib/rbnacl/libsodium.rb

@@ -4,13 +4,13 @@ module RbNaCl
   module Libsodium
     sodiumlib_dir = File.expand_path("../../../vendor/libsodium/dist/lib/", __FILE__)
 
-    sodiumlib_filename = case RUBY_DESCRIPTION
-    when /darwin/ then "libsodium.dylib"
-    when /Windows|(win|mingw)32/ then "libsodium.dll"
-    else "libsodium.so"
+    sodiumlib_glob = case RUBY_DESCRIPTION
+    when /darwin/ then "libsodium*.dylib"
+    when /Windows|(win|mingw)32/ then "libsodium*.dll"
+    else "libsodium*.so"
     end
 
-    ::RBNACL_LIBSODIUM_GEM_LIB_PATH = File.join(sodiumlib_dir, sodiumlib_filename)
+    ::RBNACL_LIBSODIUM_GEM_LIB_PATH = Dir.glob(File.join(sodiumlib_dir, sodiumlib_glob)).first
   end
 end