Check for Rails before using __in_erb_template

The tests all load (some version of) Rails. Haml is not always going to
be used in a Rails environment, and ap_ge3? will be false in these
situations, so __in_erb_template will be used, generating
needless warnings.

Check that Rails is loaded, as well as it being an earlier version,
before using __in_erb_template in generated code.

lib/haml/compiler.rb

@@ -14,7 +14,7 @@ extend Haml::Helpers
 _hamlout = @haml_buffer = Haml::Buffer.new(haml_buffer, #{options_for_buffer.inspect})
 _erbout = _hamlout.buffer
 END
-      preamble << "__in_erb_template = true;" unless ap_geq_3?
+      preamble << "__in_erb_template = true;" if rails_env && !ap_geq_3?
       postamble = <<END.gsub("\n", ";")
 #{precompiled_method_return_value}
 ensure