Bump Elixir requirement to 1.7

.travis.yml

@@ -10,15 +10,6 @@ sudo: false
 script: mix test
 matrix:
   include:
-    - elixir: 1.5
-      otp_release: 19.3
-    - elixir: 1.5
-      otp_release: 20.3
-    - elixir: 1.6
-      otp_release: 19.3
-    - elixir: 1.6
-      otp_release: 20.3
-    - elixir: 1.6
       otp_release: 21.0
     - elixir: 1.7
       otp_release: 19.3

mix.exs

@@ -9,7 +9,7 @@ defmodule Httparrot.Mixfile do
     [
       app: :httparrot,
       version: "1.2.0",
-      elixir: "~> 1.5",
+      elixir: "~> 1.7",
       name: "HTTParrot",
       description: @description,
       package: package(),