Fix Python detection for Python 3.x

  File "<string>", line 1
    import sys; print sys.version_info[:2]
                        ^
SyntaxError: invalid syntax

.travis.yml

@@ -8,6 +8,6 @@ python:
 install:
     - python setup.py -q install
 script:
-    - if [[ "$(python -c 'import sys; print sys.version_info[:2]')" == "(2, 6)" ]]; then pip install unittest2; fi
+    - if [[ "$(python -c 'import sys; print(sys.version_info[:2])')" == "(2, 6)" ]]; then pip install unittest2; fi
     - py.test --tb=native jsonschema
     - python -m doctest README.rst