Fix test failure on `rubygems-head`

There's a regression in which RubyGems ignore the version of the
packaged gem file on `rubygems-head`. This is a workaround to make sure
that RubyGems install a correct version of the test gem.

See also https://github.com/rubygems/rubygems/issues/875
Do not display error when requiring rake task

Instead, only show missing Appraisals file error when user execute
`bin/appraisal`.