Disable autorun when running RSpec

N.B. This pull request doesn't contain any tests.  I couldn't see 
any for the rspec command, but would be happy to add one if
pointed in the right direction.

Current versions of rspec-rails generate a spec_helper.rb file that
requires 'rspec/autorun'.  This allows individual specs to be run
using `ruby -Ispec path/to/spec`, but causes the spring `rspec`
command to run specs twice.  Disabling autorun before requiring
preloaded files (which will normally include spec_helper) prevents
this.