Remove pip installation from Travis CI

.travis.yml

 language: python
+
 python:
   - "2.6"
   - "2.7"
-# command to install dependencies
-install:
-  - pip install yara-python
+
+install: 
   - python setup.py install
-# command to run tests
-script: python tests.py
+
+script: 
+  - python tests.py