Fix for undefined method `start_with?' for nil:NilClass in Rakefile

There's a minor issue when attempting to run any rake task that does not contain any arguments:

<pre>
bundle exec rake -T

rake aborted!
undefined method `start_with?' for nil:NilClass
/Users/dtykocki/Development/ruby/rstat.us/Rakefile:5:in `<top (required)>'
/Users/dtykocki/.rvm/gems/ruby-1.9.3-p374@rstatus/bin/ruby_noexec_wrapper:14:in `eval'
/Users/dtykocki/.rvm/gems/ruby-1.9.3-p374@rstatus/bin/ruby_noexec_wrapper:14:in `<main>'
(See full trace by running task with --trace)

</pre>