In the Houdini bin folder? Where, in the main Houdini installation folder? This is not a good place for the plugin at all.
I've just checked the demo version and it is right, it is the 3.05.3.0 version. I don't know how, but somehow you have an older plugin version in your system, probably in your $HOME/houdini15.5 folder.
I think that you should delete the $HOME/houdini15.5 folder, and be sure that you don't have the plugin in any place, including your main Houdini bin folder, if you have dropped it there.
Later follow the installation instructions: extract the plugin folder from the .zip file (the "Octane_3.05.3.0_Houdini_15.5.673_demo_Win64" folder), put it in any place on your hard disk, and add the plugin config info to the houdini.env file to point to the plugin directory.
Hope this helps,
-Juanjo