Bump minimal required Elixir version

Since PR #514, the [`Kernel.then/2`
macro](https://hexdocs.pm/elixir/Kernel.html#then/2) is used, which was
introduced in Elixir 1.12.0. If installed on older Elixir versions, it
will fail to compile with the error below.

```
==> dialyxir
Compiling 66 files (.ex)

== Compilation error in file lib/dialyxir/project.ex ==
** (CompileError) lib/dialyxir/project.ex:365: undefined function then/2
    (elixir) expanding macro: Kernel.|>/2
    lib/dialyxir/project.ex:365: Dialyxir.Project (module)
    (elixir) expanding macro: Kernel.if/2
    lib/dialyxir/project.ex:365: Dialyxir.Project (module)
```

Bump the minimal required Elixir version to communicate older versions
are no longer supported. Elixir 1.12 is also the oldest version tested
in the CI.

mix.exs

@@ -8,7 +8,7 @@ defmodule Dialyxir.Mixfile do
     [
       app: :dialyxir,
       version: @version,
-      elixir: ">= 1.6.0",
+      elixir: ">= 1.12.0",
       elixirc_paths: elixirc_paths(Mix.env()),
       description: description(),
       package: package(),