Only set BUNDLE_GEMFILE from Rakefile when not set

and then only when not running in Appraisal or
on TravisCI
Ensure Travis bundles gems and runs bundle exec
Continue test when ruby-debug is missing
Prevent ruby-debug from being installed on TravisCI

The 'local_development' group is a way to specify gems
you may use while developing the gem, but aren't required
to run the tests.
Remove duplicate mime-types dep from Gemfile

The gemspec already specifies a version of mime-types.
Specifying it in the Gemfile is redundant and repetitive. :)
Widen mime-types dep to include 2.x
Update TravisCI configs to current format
Run tests against various mime-types using Appraisal

Run tests against all mime-types version with

    appraisal rake

or

    rake appraisal
Configure TravisCI to run on all gemfiles

Exclude invalid ruby/mime-types combos from TravisCI
Add instructions on running appraisal or wwtd
Correct gemfile path on TravisCI

It looks like when @travisci doesn't find the
expected gemfile, it just skips bundling altogether,
but continues with no error message.

This leads to confusing failures.