Update pushover user configuration

This is related to https://github.com/Seldaek/monolog/commit/46cedf5e2f0553d32037fe6e275bff8aec0bb5f3. Using an array of users will need version 1.6+ of monolog.
Merge pull request #48 from EmmanuelVella/patch-1

Allow an array of users for pushover config