update travis plugin

Author: chfw

.travis.yml

@@ -11,6 +11,7 @@ install:
   - pip install git+https://github.com/chfw/ezodf.git
   - pip install git+https://github.com/chfw/pyexcel-io.git
   - pip install git+https://github.com/chfw/pyexcel.git
+  - pip install git+https://github.com/chfw/pyexcel-xls.git
   - pip install -r requirements.txt --use-mirrors
   - pip install -r tests/requirements.txt --use-mirrors
 script: