Handle non-ascii characters in platform version shown in Versions panel.

Non-ascii characters in platform name, for example Fedora's <<Schrödinger’s Cat>>
causes VersionDebugPanel to fail with UnicodeDecodeError. Added compat.text_(.., 'utf8')
convertion and moved platform name discovery to helper method to be easily swaped in unit tests.
Merge branch 'master' of github.com:hanula/pyramid_debugtoolbar into hanula-master
add changelog entry