need prometheus_ex 3.0.2 for elixir 1.7

[whenceforth]

prometheus_ex doesn't compile under elixir 1.7 due to https://github.com/deadtrickster/prometheus.ex/issues/18

That's been fixed as of version 3.0.2 of prometheus_ex, so an upgrade of the version here would probably solve it for prometheus-phoenix.

[deadtrickster]

Yes, but ~> operator used here (~> 3.0) so should download the latest 3.x.x version. I could remove 'ors' in the mix.exs but this will force everyone to update to 3.x.x. Need to think about it more. In meantime please use override.