Rework the way latest template is selected

Current problem: `#latest_template` indiscriminately selects a template matching any given string. This effectively nullifies the use of `-n` flag; if we have images `travis-ruby-*` and `travis-update-ruby-*`, we cannot predict which one we will get.

Here, we

1. rename `#latest_template` to `#latest_template_matching` to make intentions clear
2. introduce #latest_released_template so that the absence of prefix (added when `-n` is used to create the template) signifies the 'release' status
3. tweak the `boot` Thor task so that `-n` can restrict the template selection as intended; `thor travis:images:boot ruby -n update -a pro` will boot the latest image with the name which starts with `travis-update-ruby` in the 'pro' account.
Since the argument is named `regexp`, I'd expect it to already be a `Regexp` object.