Add support for igniter 0.6

[wingyplus]

I try installing mneme with igniter 0.6 (in my project) and got:

mix deps.get returned exited with code: `1`

** (RuntimeError) Resolving Hex dependencies...
Resolution completed in 0.226s
Because "mneme >= 0.10.2" depends on "igniter ~> 0.3.76 or ~> 0.4.0 or ~> 0.5.0" and "mneme >= 0.10.0 and < 0.10.2" depends on "igniter ~> 0.3.76 or ~> 0.4.0", "mneme >= 0.10.0" requires "igniter ~> 0.3.76 or ~> 0.4.0 or ~> 0.5.0".
And because "your app" depends on "igniter ~> 0.6", "mneme >= 0.10.0" is forbidden.
So, because "your app" depends on "mneme ~> 0.10", version solving failed.
** (Mix) Hex dependency resolution failed

So I add it to the mix.exs to ensure it works with igniter 0.6.

[tcoopman]

@zachallaun any chance you can merge this and push a release?