[tests] Use rspec's ruby executable for run

This fixes the assumption that `ruby` is always available from PATH.

Signed-off-by: David Crosby <[email protected]>

spec/mixlib/shellout_spec.rb

@@ -18,7 +18,7 @@
   let(:ruby_code) { raise "define let(:ruby_code)" }
   let(:options) { nil }
 
-  let(:ruby_eval) { lambda { |code| "ruby -e '#{code}'" } }
+  let(:ruby_eval) { lambda { |code| "#{Gem.ruby} -e '#{code}'" } }
 
   context "when instantiating" do
     subject { shell_cmd }
@@ -1177,7 +1177,7 @@ def shell_out_cmd
 
         context "on unix", :unix_only do
           def ruby_wo_shell(code)
-            parts = %w{ruby}
+            parts = [ Gem.ruby ]
             parts << "-e"
             parts << code
           end