Disable autorun when running RSpec

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.
Merge pull request #98 from tomafro/disable-rspec-autorun

Disable autorun when running RSpec