Rename requirements-travis.yml to use ending .txt

.travis.yml

@@ -7,7 +7,7 @@ python:
   - "3.6"
 # command to install dependencies
 install:
-  - "pip install -r requirements-travis.yml"
+  - "pip install -r requirements-travis.txt"
 script:
   - flaskbb translations compile
   - tox