Change elixir installation process

Resolves travis-ci/travis-ci#3269

If the desired Elixir version is not installed already,
pull down package from s3.hex.pm, and set up Elixir runtime
as kiex expects.

'kiex list' does not make network calls to GitHub, so that
we should not reach GitHub's API rate limit.

lib/travis/build/script/elixir.rb

@@ -6,6 +6,8 @@ class Elixir < Erlang
           elixir: '1.0.2',
           otp_release: '17.4'
         }
+        KIEX_ELIXIR_HOME = '$HOME/.kiex/elixirs/'
+        KIEX_MIX_HOME    = '$HOME/.kiex/mix/'
 
         def export
           super
@@ -14,15 +16,19 @@ def export
 
         def announce
           super
-          sh.fold "kiex" do
-            sh.cmd "kiex list | grep -F #{elixir_version} >/dev/null", echo: false
-            sh.if "$? -eq 0" do
-              sh.echo "Using Elixir #{elixir_version}", ansi: :yellow
-            end
-            sh.else do
-              sh.echo "Installing Elixir #{elixir_version}", ansi: :yellow
-              sh.cmd "travis_retry kiex install #{elixir_version} && kiex use #{elixir_version}", assert: true, timing: true
+
+          sh.fold "elixir" do
+            sh.cmd "kiex list | grep #{Regexp.escape(elixir_version).shellescape}", echo: false, assert: false
+            sh.if "$? != 0" do
+              sh.echo "Installing Elixir #{elixir_version}"
+              sh.cmd "wget http://s3.hex.pm/builds/elixir/v#{elixir_version}.zip", assert: true, timing: true
+              sh.cmd "unzip -d #{KIEX_ELIXIR_HOME}/elixir-#{elixir_version} v#{elixir_version}.zip 2>&1 > /dev/null", echo: false
+              sh.cmd "echo 'export ELIXIR_VERSION=#{elixir_version}
+export PATH=#{KIEX_ELIXIR_HOME}elixir-#{elixir_version}/bin:$PATH
+export MIX_ARCHIVES=#{KIEX_MIX_HOME}elixir-#{elixir_version}' > #{KIEX_ELIXIR_HOME}elixir-#{elixir_version}.env"
             end
+
+            sh.cmd "kiex use #{elixir_version}"
           end
           sh.cmd "elixir --version"
         end