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.
Tweak latest_template_matching based on feedback
Pass a String instead of a Regexp
Merge pull request #9 from travis-ci/ha-bug-latest_template_selection

Rework the way latest template is selected