Fix: use new serverspec spec helper

Serverspec 2.0 introduced a change to the spec_helper.

See: http://serverspec.org/changes-of-v2.html for more info.
Add: Test for ruby-switch

There are Ruby 2.2 packages in the brightbox repo.  At present they will
be installed, and set to 2.2, which will cause all sorts of problems
because there are no dev packages yet.  Also we don't want to use 2.2.

Ruby-switch is a simple interface to the alternatives system and allows
us to specify that we should use 2.1 even if something else gets installed.

This tests that the tool is installed and that 2.1 is the default.
Fix: Update command tests to use correct matchers

Serverspec 2.0 doesn't accept the stdout methods.  Instead we
must assert that the content matches.
Add: chefspec test for ruby-switch
Fix: make ruby version check less strict
Fix: Use ruby-switch to fix Ruby to 2.1

Brightbox are publishing a 2.2-preview package which seems to be
installed if you install 2.1, and set to the default.

This fix installs the ruby-switch command and uses it to set the
version to 2.1.
Bump version by a patch
Merge pull request #2 from Atalanta/ruby-switch

Ruby switch