prepare release 2.3.1

prepare release 2.3.0 (psf#116)

prepare release 2.2.0

Prepare release 2.1

Update Travis CI URL.

Prepare 2.0 release

Prepare 1.7.1 release

Update changelog; set version to 1.7.0

Change the classifier to: Development Status :: 5 - Production/Stable

prepare 1.6.1 release

Add IBM Z support to collect_cpu_freq()

prepare 1.5.1 release