refactor: only define test repo if sqlite3 is available (#67)

This keeps the compiler happy if we're in a project or environment in
which the user has chosen not to opt in to sqlite3 usage.

lib/snowflex/sqlite_test_repo.ex

@@ -1,5 +1,7 @@
-defmodule Snowflex.SQLiteTestRepo do
-  use Ecto.Repo,
-    otp_app: :snowflex,
-    adapter: Ecto.Adapters.SQLite3
+if Code.ensure_loaded?(Ecto.Adapters.SQLite3) do
+  defmodule Snowflex.SQLiteTestRepo do
+    use Ecto.Repo,
+      otp_app: :snowflex,
+      adapter: Ecto.Adapters.SQLite3
+  end
 end

mix.exs

@@ -54,7 +54,7 @@ defmodule Snowflex.MixProject do
       {:dialyxir, "~> 1.0", only: :dev, runtime: false},
       {:ex_doc, ">= 0.0.0", only: :dev, runtime: false},
       {:meck, "~> 0.9", only: :test},
-      {:ecto_sqlite3, "~> 0.8.2", only: [:dev, :test]}
+      {:ecto_sqlite3, "~> 0.8.2", optional: true, only: [:dev, :test]}
     ]
   end