Rakefile: Remove "git pull" when deploying

Current "rake deploy" doesn't handle a situation: User
uses Octopress from multiple places. That's because we
do "rake setup_github_pages" in different machines, which
yields different history on _deploy repo(at least there is
a "Octopress init" commit which is different in these machines).

So I remove the "git pull" from "rake deploy", and force the push
everytime. This blows the commit history of _deploy at all,
but actually we don't need this history. This makes we can work
on our blog from different machines.

Signed-off-by: Mark Zhang <markz@nvidia.com>