travis/mix: test for previous versions

Test for previous versions of Elixir (1.7.4) and Erlang (21.2.6) to know
if this script would run with the available Erlang's package on Debian's
Package Repository.

.travis.yml

@@ -4,3 +4,7 @@ otp_release: "22.3"
 env:
   - MIX_ENV=test
 script: mix coveralls.travis
+jobs:
+  include:
+    - elixir: "1.7.4"
+      otp_release: "21.2.6"

mix.exs

@@ -6,7 +6,7 @@ defmodule ExTldr.MixProject do
       app: :ex_tldr,
       description: description(),
       version: "0.2.0",
-      elixir: "~> 1.9",
+      elixir: "~> 1.7",
       start_permanent: Mix.env() == :prod,
       deps: deps(),
       escript: [main_module: ExTldr, name: "extldr"],