Fix error in case schema dir is not set

Since version 1.2.1 I have following error, because schema_dir raises exception when env['HOOKUP_SCHEMA_DIR'] is nil.

```
/Users/brainopia/.rvm/gems/ruby-1.9.3-p374@ohmystats/gems/hookup-1.2.1/lib/hookup.rb:82:in `expand_path': can't convert nil into String (TypeError)
	from /Users/brainopia/.rvm/gems/ruby-1.9.3-p374@ohmystats/gems/hookup-1.2.1/lib/hookup.rb:82:in `schema_dir'
	from /Users/brainopia/.rvm/gems/ruby-1.9.3-p374@ohmystats/gems/hookup-1.2.1/lib/hookup.rb:114:in `initialize'
	from /Users/brainopia/.rvm/gems/ruby-1.9.3-p374@ohmystats/gems/hookup-1.2.1/lib/hookup.rb:70:in `new'
	from /Users/brainopia/.rvm/gems/ruby-1.9.3-p374@ohmystats/gems/hookup-1.2.1/lib/hookup.rb:70:in `post_checkout'
	from /Users/brainopia/.rvm/gems/ruby-1.9.3-p374@ohmystats/gems/hookup-1.2.1/lib/hookup.rb:27:in `run'
	from /Users/brainopia/.rvm/gems/ruby-1.9.3-p374@ohmystats/gems/hookup-1.2.1/lib/hookup.rb:12:in `run'
	from /Users/brainopia/.rvm/gems/ruby-1.9.3-p374@ohmystats/gems/hookup-1.2.1/bin/hookup:4:in `<top (required)>'
	from /Users/brainopia/.rvm/gems/ruby-1.9.3-p374@ohmystats/bin/hookup:19:in `load'
	from /Users/brainopia/.rvm/gems/ruby-1.9.3-p374@ohmystats/bin/hookup:19:in `<main>'
	from /Users/brainopia/.rvm/gems/ruby-1.9.3-p374@ohmystats/bin/ruby_noexec_wrapper:14:in `eval'
	from /Users/brainopia/.rvm/gems/ruby-1.9.3-p374@ohmystats/bin/ruby_noexec_wrapper:14:in `<main>'
```