specify supported Python versions in setup.py

As suggested by hugovk in gcovr#195.

The classifiers now list all versions tested by Travis CI. This
excludes 2.6.

The "python_requires" setting is the minimum allowed version, and will
be checked by pip during installation.

setup.py

@@ -34,6 +34,7 @@ def read(*rnames):
       url='http://gcovr.com',
       license='BSD',
       platforms=["any"],
+      python_requires='>=2.6',
       description='A Python script for summarizing gcov data.',
       long_description=read('README.txt'),
       classifiers=[
@@ -45,8 +46,11 @@ def read(*rnames):
           'Operating System :: Microsoft :: Windows',
           'Operating System :: Unix',
           'Programming Language :: Python',
+          'Programming Language :: Python :: 2.7',
+          'Programming Language :: Python :: 3.4',
+          'Programming Language :: Python :: 3.5',
           'Programming Language :: Unix Shell',
-          'Topic :: Software Development :: Libraries :: Python Modules'
+          'Topic :: Software Development :: Libraries :: Python Modules',
       ],
       packages=['gcovr'],
       keywords=['utility'],