gnu: Add ruby-unf.

* gnu/packages/ruby.scm (ruby-unf): New variable.

gnu: Add ruby-shoulda.

* gnu/packages/ruby.scm (ruby-shoulda): New variable.

gnu: Add ruby-shoulda-matchers-2.

* gnu/packages/ruby.scm (ruby-shoulda-matchers-2): New variable.

gnu: Add ruby-shoulda-matchers.

* gnu/packages/ruby.scm (ruby-shoulda-matchers): New variable.

gnu: Add ruby-shoulda-context.

* gnu/packages/ruby.scm (ruby-shoulda-context): New variable.

gnu: Add ruby-sqlite3.

* gnu/packages/ruby.scm (ruby-sqlite3): New variable.

gnu: Add ruby-mime-types.

* gnu/packages/ruby.scm (ruby-mime-types): New variable.

gnu: Add ruby-fivemat.

* gnu/packages/ruby.scm (ruby-fivemat): New variable.

gnu: Add ruby-mime-types-data.

* gnu/packages/ruby.scm (ruby-mime-types-data): New variable.

gnu: Add ruby-minitest-rg.

* gnu/packages/ruby.scm (ruby-minitest-rg): New variable.

gnu: Add ruby-minitest-bonus-assertions.

* gnu/packages/ruby.scm (ruby-minitest-bonus-assertions): New variable.

gnu: Add ruby-minitest-moar.

* gnu/packages/ruby.scm (ruby-minitest-moar): New variable.

gnu: Add ruby-minitest-pretty-diff.

* gnu/packages/ruby.scm (ruby-minitest-pretty-diff): New variable.

gnu: Add ruby-minitest-focus.

* gnu/packages/ruby.scm (ruby-minitest-focus): New variable.

gnu: Add ruby-turn.

* gnu/packages/ruby.scm (ruby-turn): New variable.

gnu: Add aria2.

* gnu/packages/bittorrent.scm (aria2): New variable.

gnu: c-ares: Move to adns.scm.

* gnu/packages/aria2.scm (c-ares): Move to...
* gnu/packages/adns.scm (c-ares): ... here.
* gnu/packages/aria2.scm: Remove file.
* gnu-system.am (GNU_SYSTEM_MODULES): Remove aria2.scm.

gnu: libressl: Update to 2.2.5.

* gnu/packages/tls.scm (libressl): Update to 2.2.5.

gnu: nginx: Update to 1.8.1 [fixes CVE-2016-{0742,0746,0747}].

* gnu/packages/web.scm (nginx): Update to 1.8.1.

doc: Show how to boot result of 'vm-image'.

* doc/guix.texi (Running GuixSD in a VM): New node.
(Invoking guix system): Add reference to 'Running GuixSD in a VM'.