Add summary option to 'show' cli command

Add summary option to 'show' cli command

Add summary option to 'show' cli command
Avoid installing C exts on JRuby to allow running specs.

This patch does the following:

* Omits rdiscount and ronn from deps to be installed when running
  on JRuby, since they're only needed for release tasks.
* Removes an unnecessary require of ronn.
* Rewrites release tasks to warn that ronn is required if it
  cannot be loaded.

The result is that JRuby can run both spec:deps and spec targets,
bringing us closer to getting specs green on JRuby.
Update badges in README to use Shields

http://shields.io/ provides many standardized badges for OSS. The PNG versions are similar to the images currently used, but the SVG versions added here are more legible on high pixel density displays.
Avoid a NameError when OpenSSL is unavailable.

Fixes #2893.
Use documentation rspec formater
Merge pull request #2900 from TimMoore/master

Use documentation rspec formater
Revert "Merge pull request #2886 from mdkent/issue-2885"

This reverts commit bb8f694147cb07cd820297978b0378ffa4c6d54b, reversing
changes made to b12f33eed564f881c8a5038bd8de9be10c6e664e.
Fail faster when git is not installed.
Only retry git commands that hit the network.
Merge pull request #2899 from TimMoore/git-retry-revisited

Git retry revisited
Merge pull request #2896 from seanlinsley/patch-1

Update badges in README to use Shields

[ci skip]
Merge pull request #2898 from TimMoore/issue-2893-fix-gem-security-policies-name-error

Avoid a NameError when OpenSSL is unavailable.
Merge pull request #2868 from lardcanoe/add_summary_to_show

Display gem summary when verbose is passed into show command
Merge pull request #2895 from headius/specs_on_jruby

Avoid installing C exts on JRuby to allow running specs.

Conflicts:
	Rakefile