fix typo

.travis.yml

@@ -6,6 +6,6 @@ python:
 before_install:
   - sudo apt-get install -y libxml2-dev libxslt1-dev zlib1g-dev libffi-dev libyaml-dev libssl-dev
 
-install: "pip install -r dev-requirements"
+install: "pip install -r dev-requirements.txt"
 
 script: nosetests