Using not_if block, but still continues to download

default.rb

include_recipe "nodejs::npm"

hubot_user = "#{node['hubot']['user']}"
hubot_group = "#{node['hubot']['group']}"
install_dir = "#{node['hubot']['install_dir']}"
install_dir_parent = "#{node['hubot']['install_dir_parent']}"
version = "#{node['hubot']['version']}"

# Create the hubot user to run hubot
group hubot_group do
end

user hubot_user do
  comment "Runs hubot"
  gid hubot_group
  home install_dir
  shell "/bin/bash"
end

directory install_dir do
  owner hubot_user
  group hubot_group
  mode "0775"
  action :create
end

# required os packages
%w{ libexpat1 libexpat1-dev }.each do |pkg|
  package pkg do
    action :install
  end
end

bash "install hubot" do
  user hubot_user
  group hubot_group
  environment ({'HOME' => "#{install_dir}" })
  cwd install_dir_parent
  code <<-EOH
    wget -P /tmp https://github.com/downloads/github/hubot/hubot-#{version}.tar.gz && \
    tar xzf /tmp/hubot-#{version}.tar.gz && \
    cd hubot && \
    npm install
  EOH
  not_if "#{install_dir}/bin/hubot -v 2>&1 | grep '#{version}'"
end