Various fixes and improvements in the OpenResty installer

* removed the trailing line spaces in the OpenResty installer script.
* always enable the dual number mode in OpenResty's LuaJIT build for Linux.
* specify -j2 for both ./configure and make to speed up the OpenResty build.
* specify the C compiler option -O2 for the openresty build.
* fix lettercase in the OpenResty installer.
* upgraded OpenResty to the latest formal release, 1.5.12.1.
    * we also remove the --with-luajit option from ./configure because it is already the default.