Exclude examples/ from gem file

[sue445]

This is unnecessary for running

Before

$ bundle exec rake build

$ ls -l pkg/
total 992
-rw-r--r--  1 sue445  staff  506880  1 28 01:40 rubicure-0.4.13.gem

After

$ bundle exec rake build

$ ls -l pkg/
total 80
-rw-r--r--  1 sue445  staff  38912  1 28 01:45 rubicure-0.4.13.gem