Resolve Compile Warning and raise Elixir Minimum Requirement to 1.10

[maennchen]

Resolves the following compile warning:

warning: Code.ensure_compiled?/1 is deprecated. Use Code.ensure_compiled/1 instead (see the proper disclaimers in its docs)
  lib/plug_checkup/options.ex:96: PlugCheckup.Options.validate_json_encoder!/1

Since the function Code.ensure_compiled/1 is only available since Elixir 1.10, I raised the minimum requirement.

If you prefer, we could also detect the Elixir Version and leave the old code in place for older versions.

Based on #72, merge that first.