Put braintree requirement in .travis.yml instead of requirements.txt

.travis.yml

@@ -4,7 +4,7 @@ python:
   - "3.4"
 install:
     - pip install -r requirements.txt
-    - pip install coveralls mock mysqlclient
+    - pip install braintree coveralls mock mysqlclient
 env:
   - DB=sqlite
   - DB=mysql

requirements.txt

@@ -1,4 +1,3 @@
-braintree
 django-appconf==1.0.1
 django-ses-backend==0.1.1
 Django==1.9