Drop support for Elixir 1.5

.travis.yml

@@ -1,12 +1,6 @@
 language: elixir
 matrix:
   include:
-    - otp_release: 18.3
-      elixir: 1.5
-    - otp_release: 19.3
-      elixir: 1.5
-    - otp_release: 20.0
-      elixir: 1.5
     - otp_release: 19.3
       elixir: 1.6
     - otp_release: 20.0

CHANGELOG.md

@@ -26,7 +26,7 @@ This project adheres to [Semantic Versioning](http://semver.org/) and
 
 ### Changed
 
-- Elixir 1.4 is no longer supported
+- Elixir versions < 1.6 are no longer supported
 - `RDF.String.new/2` and `RDF.String.new!/2` produce a `rdf:langString` when 
   given a language tag
 - `RDF.IRI.absolute/2` returns `nil` if the given base is not absolute, instead 

mix.exs

@@ -9,7 +9,7 @@ defmodule RDF.Mixfile do
     [
       app: :rdf,
       version: @version,
-      elixir: "~> 1.5",
+      elixir: "~> 1.6",
       build_embedded: Mix.env == :prod,
       start_permanent: Mix.env == :prod,
       deps: deps(),