Fix: Correct rsync SSH option quoting for compatibility

[pointum]

I’ve got this issue recently:

Middleman ## Deploying via rsync to [url] port=22
rsync(14072): error: exec on '': No such file or directory
rsync(12666): error: unexpected end of file
rsync(12666): error: io_read_nonblocking
rsync(12666): error: io_read_buf
rsync(12666): error: io_read_int
rsync(12666): warning: child 14072 exited with status 14

It appears that some recent macOS or zsh update enforces more strict quoting when parsing rsync arguments.

The '-e ssh -p #{port}' argument that previously worked, should be -e 'ssh -p #{port}'

I’m submitting this PR in case someone else encounters this issue, even if it never gets to be merged.