Update Rakefile to reflect renamed README.rdoc
Update Rakefile to reflect renamed README.rdoc

The file was renamed in fa28335, but the Rakefile was not updated. This
was causing all Rake tasks to raise an exception.

Tested-by: Mark Somerville <f1b5a91d4d6ad523f2610114591c007e75d15084@scottishclimbs.com> [F16, MRI 1.9.3]
Signed-off-by: Mark Somerville <f1b5a91d4d6ad523f2610114591c007e75d15084@scottishclimbs.com>

rapid7/rb-readline,abinoam/rb-readline,ConnorAtherton/rb-readline