Extract release note display into its own method

Part of #516
Test SetupCommand#show_release_notes

Part of #516
Reduce line length in #show_release_notes
Use UTF-8 when reading history for release notes

Since History.txt is a UTF-8 file it must be read as UTF-8 content.
Otherwise string operations do not work.

While this is harmless as RubyGems was installed by this point, it is
confusing to users.

Fixes #516