Update to the latest Gem

This MR updates the Gem submodule to the latest revision from https://github.com/umlaeute/Gem (rev. 01b1861, which is version 0.94 + some bugfixes).

The Gem snapshot we distribute with Purr Data is almost 3 years old now. The issues prompting the creation of https://git.purrdata.net/jwilkes/Gem have since been fixed. Gem is on Github now, it has received a lot of bugfixes, its video support has recently been vastly improved, and it even compiles on 64 bit OSX now.

I have been building and running the latest Gem version with Purr Data on both Linux and Windows for quite some time now, and it has been very stable for me. The immediate benefits we get by updating is improved video support on Linux and Windows, whereas in the old snapshot video has some bugs on Linux and doesn't work at all on Windows. It's also an important stepping stone to get Gem to work with Purr Data on modern macOS. I don't see any reason to stick to the old snapshot any more.