add v0.14.1 and v0.14.2 to test matrix
don't encode the key `nil` as `null`

fixes #31
update .travis.yml to include v1.0.0-rc2
update travis.yml for elixir v1.0.0
require jsx 2.1.1 and bump version to 2.1
update contributors in package def
semver why you gotta be so picky?
update version in README
add CONTRIBUTORS.md
bump required elixir version to 0.13.3
brand new day