Remove unneeded shell exec to get Ruby version info

In issue getsentry#943 sub-point getsentry#2, it was identified that using RUBY_DESCRIPTION
returns the same thing as `ruby -v` does for all currently supported Ruby
versions. This change fixes that sub-point and provides a fallback to the
old method in the event as a safety precaution.

lib/raven/context.rb

@@ -35,7 +35,7 @@ def os_context
       def runtime_context
         @runtime_context ||= {
           :name => RbConfig::CONFIG["ruby_install_name"],
-          :version => Raven.sys_command("ruby -v")
+          :version => RUBY_DESCRIPTION || Raven.sys_command("ruby -v")
         }
       end
     end