4th Beta of soon to be stable version

What's Changed

• Test in Elixir 1.3.X & different otp releases by @maennchen in #32
• #31 Stream get_[next|previous]_run_dates function by @maennchen in #33
• Fix Dialyzer, Remove Dead Code by @maennchen in #34

Full Changelog: v1.0.0-beta.3...v1.0.0-beta.4