.travis.yml: Add python 2.6 among the testing targets

Add a new python 2.6 target so that we can make sure
avocado works on older distros (our specific focus here
is RHEL 6, but of course, we expect the work on RHEL 6
to be useful to other distros sticking with older versions
of the python runtime).

In order to do that, we added a new 2.6 related requirements
file, and changed the requirements a bit so that it all
works under the particular Travis CI environment.

Changes from v1:
 * Per ldoktor's suggestion, use conditional install of backports
   using $TRAVIS_PYTHON_VERSION
Signed-off-by: NLucas Meneghel Rodrigues <lmr@redhat.com>

.travis.yml

 language: python
 python:
-    - "2.7"
+    - "2.7_with_system_site_packages"
+    - "2.6"
 
 branches:
     only:
         - master
 
-virtualenv:
-    system_site_packages: true
-
 before_install:
     - sudo apt-get update
-    - sudo apt-get -y --force-yes install python-libvirt python-lzma python-yaml
+    - sudo apt-get -y --force-yes install python-libvirt python-lzma libyaml-dev
 
 install:
     - pip install -r requirements-travis.txt
+    - if [ $TRAVIS_PYTHON_VERSION == '2.6' ]; then pip install -r requirements-travis-python26.txt; fi
 
 script:
     - inspekt lint

requirements-travis-python26.txt

+# All python 2.6 specific requirements (backports)
+argparse==1.3.0
+logutils==0.3.3
+importlib==1.0.3
+unittest2==1.0.0

requirements-travis.txt

@@ -4,5 +4,7 @@ nose==1.3.4
 pystache==0.5.4
 Sphinx==1.3b1
 flexmock==0.9.7
-inspektor==0.1.14
+inspektor==0.1.15
+pep8==1.6.2
 requests==1.2.3
+PyYAML==3.11