Don't install numpy on travis pypy build

.travis.yml

@@ -9,7 +9,7 @@ before_install:
  - "tar -xzvf nltk_data.tar.gz -C ~"
 # Install dependencies
 install:
- - "pip install numpy"
+ - if [[ $TRAVIS_PYTHON_VERSION != 'pypy' ]]; then pip install numpy; fi
  - "pip install -U ."
  - if [[ $TRAVIS_PYTHON_VERSION == '2.7' ]]; then pip install sphinx; fi
 # Run tests