[FIX] replace 'nltk' use by bs4
Install beautifulsoup4 in .travis.yml
Merge pull request #40 from guewen/bealdav-7-remove-nltk

beautifulsoup with more vegetables
Prevent to create duplicate bindings on invoices

The correction made at becc1bf2 was not working because it was
doing a 'continue' on the inner loop but not on the outer loop.