Add Pale Moon entry to install.rdf

[redyoshi49q]

As of v29.2.0, Pale Moon will no longer try to load a browser extension built for legacy Firefox unless the install.rdf also targets Pale Moon (see https://forum.palemoon.org/viewtopic.php?f=1&t=26657 ). Passifox used to work on Pale Moon without any modifications; now, it works on Pale Moon, but only when the install.rdf of the .xpi file is updated.

On my end, I was able to get a passifox.xpi working in Pale Moon after making these changes to the project's install.rdf file.