Implement Synth as an exchange rate provider (#574)

* Implement Synth as an exchange rate provider

* Add assertions to provider interface test

* Assert the correct provider error is raised

* Remove unnecessary parens

test/models/provider/synth_test.rb

@@ -0,0 +1,9 @@
+require "test_helper"
+
+class Provider::SynthTest < ActiveSupport::TestCase
+  include ExchangeRateProviderInterfaceTest
+
+  setup do
+    @subject = Provider::Synth.new
+  end
+end