R1:644b05ec08ff

Minor fix for `lipsum generate`

Summary:
I was hitting the following error when attempting to use `./bin/lipsum generate`:

```
[2015-02-28 21:55:00] ERROR 2: array_rand(): Second argument has to be between 1 and the number of elements in the array at [/usr/src/phabricator/src/applications/pholio/lipsum/PhabricatorPholioMockTestDataGenerator.php:93]
arcanist(head=master, ref.master=d8182cf55d50), phabricator(head=master, ref.master=658175925931), phutil(head=master, ref.master=260e6105dee5)
#0…