merge "pip install" commands

.travis.yml

@@ -16,8 +16,7 @@ before_install:
   - sudo systemctl restart mariadb
   - python --version
 install:
-  - pip install -r requirements.txt
-  - pip install -r tests/requirements.txt
+  - pip install -r requirements.txt -r tests/requirements.txt
 script:
   # Creating the applicative database
   - sudo mariadb -e "CREATE DATABASE $DB_NAME; CREATE USER IF NOT EXISTS '$DB_USER'@'localhost' IDENTIFIED BY '$DB_PASSWORD'; GRANT ALL PRIVILEGES ON $DB_NAME.* TO '$DB_USER'@'localhost'; FLUSH PRIVILEGES;"