Bug fix for sitespeed.io file not found

According to the docs, when running sitespeed.io-sites, you run it from the base directory of the project, but this results in the script not being able to find sitespeed.io, as it is looking in the wrong place, causing an errror:

Will analyze 2 sites
./bin/sitespeed.io-sites: line 207: bash ./sitespeed.io: No such file or directory
./bin/sitespeed.io-sites: line 207: bash ./sitespeed.io: No such file or directory