Only test the basic functionality on Python 2.6

We need Python 2.6 to keep running for EasyBuild, but for that we do
not need to support the daemon, or cloud backends.  So exclude them
from testing and hope we do not run into packages that lack Py 2.6
support for the base functionality...

.travis.yml

@@ -1,15 +1,21 @@
 language: python
-python:
-  - "2.6"
-  - "2.7"
 addons:
   apt:
     packages:
     - time
+
+matrix:
+  include:
+    - python: "2.7"
+      env: FEATURES=openstack,ec2,daemon
+    - python: "2.6"
+      env: FEATURES=
+
 install:
   - "pip install --upgrade setuptools pip"
   - "pip install 'pytest==3.2.5' mock"
-  - "pip install -e .[openstack,ec2,daemon]"
+  - "pip install -e .[$FEATURES]"
+
 script:
   - export BOTO_CONFIG=/dev/null
   - pytest -v