Issue where b:octopress_rake_executable is not defined after "new_post" executed

Fixes issue where b:octopress_rake_executable not defined occurred after executing :Octopress new_post (presumably because of a reload?)

I'm seeing this on Vim 7.3, iTerm2 1.0.0.20130319, and zsh using oh-my-zsh.  Reproduction steps:

:Octopress new_post My example post
:Octopress generate

-> b:octopress_rake_executable not defined.

Not sure if it's kosher to use g:octopress_rake_executable here, but it fixes my problem.