Bump rainbow gem back to 2.0

Fixes crash:
```
$ gt ssh -r backend -e production
No instance specified, querying list.
0: i-1482b51c
1: i-c3b531f4
2: i-3b3ff40c
3: i-42e1d34a
4: i-c0784ec8
5: i-d1a999d9
6: i-c2f0c2ca
/Users/giacomobagnoli/.rvm/gems/ruby-1.9.3-p547/gems/gaptool-client-0.5.4/lib/gaptool-client.rb:151:in `execute': undefined method `Rainbow' for #<Gaptool::SshCommand:0x007fe1fc539f90> (NoMethodError)
	from /Users/giacomobagnoli/.rvm/gems/ruby-1.9.3-p547/gems/clamp-0.6.3/lib/clamp/command.rb:67:in `run'
	from /Users/giacomobagnoli/.rvm/gems/ruby-1.9.3-p547/gems/clamp-0.6.3/lib/clamp/subcommand/execution.rb:11:in `execute'
	from /Users/giacomobagnoli/.rvm/gems/ruby-1.9.3-p547/gems/clamp-0.6.3/lib/clamp/command.rb:67:in `run'
	from /Users/giacomobagnoli/.rvm/gems/ruby-1.9.3-p547/gems/clamp-0.6.3/lib/clamp/command.rb:125:in `run'
	from /Users/giacomobagnoli/.rvm/gems/ruby-1.9.3-p547/gems/gaptool-client-0.5.4/lib/gaptool-client.rb:382:in `<top (required)>'
	from /Users/giacomobagnoli/.rvm/rubies/ruby-1.9.3-p547/lib/ruby/site_ruby/1.9.1/rubygems/core_ext/kernel_require.rb:55:in `require'
	from /Users/giacomobagnoli/.rvm/rubies/ruby-1.9.3-p547/lib/ruby/site_ruby/1.9.1/rubygems/core_ext/kernel_require.rb:55:in `require'
	from /Users/giacomobagnoli/.rvm/gems/ruby-1.9.3-p547/gems/gaptool-client-0.5.4/bin/gt:16:in `<top (required)>'
	from /Users/giacomobagnoli/.rvm/gems/ruby-1.9.3-p547/bin/gt:23:in `load'
	from /Users/giacomobagnoli/.rvm/gems/ruby-1.9.3-p547/bin/gt:23:in `<main>'
	from /Users/giacomobagnoli/.rvm/gems/ruby-1.9.3-p547/bin/ruby_executable_hooks:15:in `eval'
	from /Users/giacomobagnoli/.rvm/gems/ruby-1.9.3-p547/bin/ruby_executable_hooks:15:in `<main>'
```

With Rainbow-2.0.0:

```
$ gt ssh -r backend -e production
No instance specified, querying list.
0: i-1482b51c
1: i-c3b531f4
2: i-3b3ff40c
3: i-42e1d34a
4: i-c0784ec8
5: i-d1a999d9
6: i-c2f0c2ca
Select a node:
```