fix: allow httpoison 2.0

[hpopp]

Addresses #234

[prkhrkat]

I'm. still getting this -

(base) prakhar@prakhar-katiyar-MacBook-Pro medqcx % mix deps.update --all
Resolving Hex dependencies...
#Incompatibility<#Term, cause: {:conflict, #Incompatibility<#Term= 1.1.5>, cause: {:conflict, #Incompatibility<#Term= 1.1.5>, #Term<not httpoison ~> 0.7 or ~> 1.0>, cause: :dependency>, #Incompatibility<#Term, #Term<not httpoison ~> 2.0>, cause: :dependency>}>, #Incompatibility<#Term, #Term<not pigeon ~> 2.0.0-rc.1>, cause: :dependency>}>
Resolution completed in 0.13s
Because pigeon >= 1.1.5 depends on httpoison ~> 0.7 or ~> 1.0 and your app depends on httpoison ~> 2.0, pigeon >= 1.1.5 is forbidden.
So, because your app depends on pigeon ~> 2.0.0-rc.1, version solving failed.
** (Mix) Hex dependency resolution failed