Use File.expand_path in script/server.

Be compatible with Ruby 1.9 and later.

Before fix: 
```
$ ruby -v
ruby 1.9.3p392 (2013-02-22 revision 39386) [x86_64-darwin11.4.2]
$ bundle exec script/server
script/server:2:in `require': cannot load such file -- script/../config/boot (LoadError)
	from script/server:2:in `<main>'
```