Added requirements.txt

.travis.yml

@@ -9,7 +9,7 @@ python:
   - nightly # 3.6
 
 install:
-#- pip install -r requirements.txt
+- pip install -r requirements.txt
 - pip install coveralls
 
 script: make travis

requirements.txt

@@ -0,0 +1 @@
+pyyaml