How to install gem/plugin in rails3 when not found on rubygems?

I want to install the simple-private-messages gem (http://agilewebdevelopment.com/plugins/开发者_StackOverflow社区simple_private_messaging), https://github.com/jongilbraith/simple-private-messages.

I added to my Gemfile:

gem 'simple-private-messages'

But get error:

Fetching source index for http://rubygems.org/
Could not find gem 'simple-private-messages (>= 0)' in any of the gem sources listed in your Gemfile.

I then tried modifying Gemfile to:

gem 'simple-private-messages', :git => 'git://github.com/professionalnerd/simple-private-messages.git'

but get error:

Fetching git://github.com/professionalnerd/simple-private-messages.git
github.com[0: 207.97.227.239]: errno=Operation timed out
fatal: unable to connect a socket (Operation timed out)
Git error: command `git clone 'git://github.com/professionalnerd/simple-private-    messages.git' "/Users/homanchou/.rvm/gems/ruby-1.9.2-p180/cache/bundler/git/simple-private-messages-26451211e7d7acc0c1b523bad2621cb2cd38f77e" --bare --no-hardlinks` 

1) How else can I install this gem locally?

2) How can I deploy this app to heroku and protect against deployment error if Heroku can not find the gem in any repositories? Do I vendor them into the app?

It doesn't look like that project is a Gem; it's actually a Rails Plugin.

To install, run this in your project folder:

rails plugin install git://github.com/jongilbraith/simple-private-messages.git