Fixed task :clear_crontab

Task fails with error:
cap aborted!
SSHKit::Runner::ExecuteError: Exception while executing on host example.com: Exception while executing on host example.com: no block given (yield)
/home/ekho/.gem/ruby/1.9.1/gems/whenever-0.9.2/lib/whenever/tasks/whenever.rake:11:in \`block in setup_whenever_task'
/home/ekho/.gem/ruby/1.9.1/gems/sshkit-1.5.1/lib/sshkit/backends/netssh.rb:54:in \`instance_exec'
/home/ekho/.gem/ruby/1.9.1/gems/sshkit-1.5.1/lib/sshkit/backends/netssh.rb:54:in \`run'
/home/ekho/.gem/ruby/1.9.1/gems/sshkit-1.5.1/lib/sshkit/runners/parallel.rb:13:in \`block (2 levels) in execute'
SSHKit::Runner::ExecuteError: Exception while executing on host example.com: no block given (yield)
/home/ekho/.gem/ruby/1.9.1/gems/whenever-0.9.2/lib/whenever/tasks/whenever.rake:11:in \`block in setup_whenever_task'
/home/ekho/.gem/ruby/1.9.1/gems/sshkit-1.5.1/lib/sshkit/backends/netssh.rb:54:in \`instance_exec'
/home/ekho/.gem/ruby/1.9.1/gems/sshkit-1.5.1/lib/sshkit/backends/netssh.rb:54:in \`run'
/home/ekho/.gem/ruby/1.9.1/gems/sshkit-1.5.1/lib/sshkit/runners/parallel.rb:13:in \`block (2 levels) in execute'
LocalJumpError: no block given (yield)
/home/ekho/.gem/ruby/1.9.1/gems/whenever-0.9.2/lib/whenever/tasks/whenever.rake:11:in \`block in setup_whenever_task'
/home/ekho/.gem/ruby/1.9.1/gems/sshkit-1.5.1/lib/sshkit/backends/netssh.rb:54:in \`instance_exec'
/home/ekho/.gem/ruby/1.9.1/gems/sshkit-1.5.1/lib/sshkit/backends/netssh.rb:54:in \`run'
/home/ekho/.gem/ruby/1.9.1/gems/sshkit-1.5.1/lib/sshkit/runners/parallel.rb:13:in \`block (2 levels) in execute'