Due to the rewrite of the plugin's core sync loop and the change for the
Notify backend on MacOS, along with all the other changes in 7.4.0, it
makes sense for us to use a release candidate before actually cutting a
proper `7.4.0` release.
This modifies Rojo's build script to throw a fit if we're building a
plugin with a semver incompatible version. In the process, it moves the
version of the plugin to a file named `Version.txt` that's parsed at
runtime. This should be minimally invasive but it's technically worse
for performance than the hardcoded table and string we had before.
This feels better than a CI check or just manually verifying because it
makes it physically impossible for us to forget since Rojo won't build
with it being wrong.
