Osxfuse: Unsigned Kexts Caveat

Related to [this
issue](https://github.com/Homebrew/homebrew/issues/31164). Adds a
warning in the “Caveats” that once Yosemite is released it is likely
you’ll need to disable the kext check mechanism for OSXFuse to
successfully run.
osxfuse: update 2.7.0 bottle.