fixes compilation error on v0.13

when compiling on Elixir v0.13 a compilation error occurs on file
lib/json/numeric.ex. This is due to the changed sigil syntax for regular
expressions.

Also the Regex.match?/2 function no longer accepts IOLists, only binary
strings.
Merge pull request #14 from patrickdet/master

fixes compilation error on Elixir v0.13