Bump version to 5.1.18.
Bump version to 5.1.18.

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