Loosen up the "slack-notifier" dependency

[loopj]

Hey Alex - could you loosen up the slack-notifier dependency in this gem?

spec.add_dependency 'slack-notifier', '~> 1.2.1'

We're using v1.5.1 of slack-notifier which is incompatible with clearbit-slack:

Bundler could not find compatible versions for gem "slack-notifier":
  In snapshot (Gemfile.lock):
    slack-notifier (= 1.5.1)

  In Gemfile:
    slack-notifier

    clearbit-slack was resolved to 0.1.0, which depends on
      slack-notifier (~> 1.2.1)

Running `bundle update` will rebuild your snapshot from scratch, using only
the gems in your Gemfile, which may resolve the conflict.

[harlow]

Hi @loopj i've bumped the version to 1.5.1. Thanks for submitting this; good to keep the gem versions current.