Potential fix for relative vs absolute path

Following instructions on `pirate_game` (https://github.com/davy/pirate_game) against Shoes4 master, found that the
`bin/shoes` script for my installed gem wouldn't run right. Although I did get the bin/shoes shell script, it would fall over on `cd ./$NEXT_DIR` when run against a fully qualified path (i.e. `/Users/jclark/.rbenv/versions/jruby-1.7.8/lib/ruby/gems/shared/gems/shoes-4.0.0.pre1/bin/shoes`).

My modified instructions of interest (your paths may vary):

```
$ git clone http://github.com/shoes/shoes4
$ git checkout master
$ gem build shoes.gemspec
$ gem install shoes-4.0.0.pre1.gem
$ alias shoes4='/Users/jclark/.rbenv/versions/jruby-1.7.8/lib/ruby/gems/shared/gems/shoes-4.0.0.pre1/bin/shoes'
$ shoes4 samples/sample27.rb
```

My bash scripting isn't awesome, so definitely keen for pointers. At the very least, seems like extracting a separate function is called for, but it's late and I wanted to post this before collapsing :sleeping: 

Also curious to confirm this will work right with the post-install option that was discussed over in #231.