Update load path modification in mspec-run.rb

This file has probably been moved at some point, the old version would to
`lib/mspec/lib` which does not exist. The new version is inspired by the syntax
of `bin/mspec-run` to keep things consistent.

Since the usual way to call this script is via `bin/mspec-run`, and that sets up
your load path, an alternative way to fix this would be to simply remove this
load path update in this file.

lib/mspec/commands/mspec-run.rb

@@ -1,6 +1,6 @@
 #!/usr/bin/env ruby
 
-$:.unshift File.expand_path(File.dirname(__FILE__) + '/../lib')
+$:.unshift File.expand_path('../../..', __FILE__)
 
 require 'mspec/version'
 require 'mspec/utils/options'