travis script is updated to use container-based environment.

Author: shtaxxx

.travis.yml

@@ -1,13 +1,19 @@
 language: python
 
+sudo: false
+
 python:
   - 2.7
   - 3.4
   - 3.5
 
+addons:
+  apt:
+    packages:
+    - iverilog
+
 install:
-  - sudo apt-get install iverilog
-  - pip install pytest pytest-pythonpath pyverilog
+  - pip install --user pytest pytest-pythonpath pyverilog
 
 script:
   - python -m pytest tests examples