Update elixir and deps

• Update library for newer Elixir/Erlang #9 Thanks @lucas-nelson ! 🎉

• Update to Elixir ~> 1.5

• Use constant time string comparison when validating signatures #6 Thanks to @lastcanal

• Bump Elixir requirement to ~> 1.2

• Relax Elixir requirement to ~> 1.0

• Release 1.0.0!
• Drop support to Elxiir < 1.0.0

• Avoid removed Enum.traverse and use for comprehension.
• Ensure support to Elixir 1.0.0.

• Handle timestamp as integer, thanks to @lastcanal
• Support ~> 0.14.3 and ~> 0.15.0

• Support 0.14.1 and 0.14.2