Change supported Elixir version to allow current Elixir master
Merge pull request #510 from guilleiguaran/bump-elixir-master-version

Change supported Elixir version to allow current Elixir master
Disables recursion in phoenix.start to fix #462.

As per discussion of issue #462, phoenix.start now
* is nonrecursive
* takes a list of routers to start as an argument
* will only attempt to guess a router in non-umbrella project
* will defer to a default list of routers provided in the mixfile
Tidied up based on review suggestions
Merge branch 'non-recursive-phoenix-start' of https://github.com/tcsc/phoenix into non-recursive-phoenix-start