fix requested node engine version in package.json

Using  ```npm install literate-programming``` causes a warning to appear 
literate-programming
```npm WARN engine literate-programming@0.8.1: wanted: {"node":">0.10"} (current: {"node":"0.10.30","npm":"1.4.21"})```

This pull-request fixes the ``engines`` parameter in the ```package.json``` file to suppress this warning.