Bump version to 2.2.0

[germsvel]

Bump version to 2.2.0. Notable changes include,

• Support for lists in sequences (PR So that we can use list sequences, fixes #223 #227)
• Using Macro.underscore/1 so that factory names don't break in elixir
  1.6 (PR Update underscore/1 to support Elixir 1.6's regex #275)

[dbernheisel]

LGTM. Verified that mix.exs already requires Elixir 1.2 which is required for Macro.underscore.

[dbernheisel]

On second-thought, can we bump it to Elixir 1.4 (from Elixir 1.2). Looks like we use List.pop_at which was introduced to Elixir 1.4. I'll open a PR.
--edit-- see #277

[germsvel]

@dbernheisel that sounds good. I'll merge this in, and we can bump the elixir version in a separate PR.