Bump version to 5.1.21.
Bump version to 5.1.21.

ardje/luaposix,jimmyharris/luaposix,rrthomas/luaposix,ardje/luaposix,snegovick/luaposix,zevv/luaposix,jimmyharris/luaposix,luaposix/luaposix,rrthomas/luaposix,snegovick/luaposix,zevv/luaposix