Use the full path location of supporting scripts.

The `script`/functions.sh file is referenced from within the Rails
sample app when it is generated in the `rspec-rails` repo. The `source`
lines do not properly adjust the file references if the `functions.sh`
script is sourced or run from another location.
Merge pull request #90 from rspec/fix-functions-script-for-rspec-rails

Use the full path location of supporting scripts.