Make building from a git archive the default

If no context is specified and we are in a git repo, then we'll build
from a git archive by default. This means we don't need a separate
setting and gives us a safer default build.

test/configuration/builder_test.rb

@@ -140,7 +140,7 @@ class ConfigurationBuilderTest < ActiveSupport::TestCase
   end
 
   test "context" do
-    assert_equal ".", @config.builder.context
+    assert_equal "-", @config.builder.context
   end
 
   test "setting context" do