Release :gem: v2.5.0
Add development dependencies for Cygwin

In the Cygwin environment, minitest and test-unit aren't bundled with
Ruby.  Add these as explicit development dependencies when running under
Cygwin.
Merge pull request #3062 from jekyll/release-2-5-0

Release v2.5.0
In the case that a Gemfile does not exist, ensure Jekyll doesn't fail.
Allow users to specify the log level via JEKYLL_LOG_LEVEL.
Merge pull request #3066 from jekyll/catch-no-gemfile
Update history to reflect merge of #3066 [ci skip]
Merge pull request #3067 from jekyll/specify-log-level
Update history to reflect merge of #3067 [ci skip]
Update site with 2 new v2.5.0 PR's.
Release :gem: 2.5.0
Merge pull request #3064 from ribbons/cygwin-env
Update history to reflect merge of #3064 [ci skip]
Something about a Travis Beta Build that @joshk mentioned