Use NLTK 2.0.6

.travis.yml

@@ -10,6 +10,7 @@ install:
   - pip install -r base-requirements.txt
   - pip install -r dev-requirements.txt
   - sudo apt-get install python-matplotlib python-numpy python-scipy python-sklearn
-  - pip install nltk==2.0.5 scikit-learn==0.12.1
+  - pip install scikit-learn==0.12.1
+  - pip install -e git+https://github.com/edx/nltk.git@2.0.6#egg=nltk
   - travis_retry ./download-nltk-corpus.sh
 script:  nosetests

requirements.txt

 -r base-requirements.txt
 scipy==0.14.0
 scikit-learn==0.12.1
-nltk==2.0.5
+-e git+https://github.com/edx/nltk.git@2.0.6#egg=nltk