Build fails on 3rd party Forked PRs

[arm4b]

Example: https://circleci.com/gh/StackStorm/st2chatops/253

This happens because of several reasons:

• Build assumes that HUBOT_SLACK_TOKEN is provided
• Build assumes that PACKAGECLOUD_TOKEN is provided

For security reasons these env variables are hidden https://circleci.com/docs/fork-pr-builds/#security-implications-of-running-builds-for-pull-requests-from-forks for forked builds. But it's possible to make some steps optional which require envs. For example HUBOT_SLACK_TOKEN is not needed for the PR builds.

For the moment it means it's hardly possible to accept 3rd party contributions.