fix: convert SourceMapGenerator to RawSourceMap

- fixes vuejs#241 and jestjs/jest#10089

Author: nogic1008

Diff for: lib/process.js

@@ -131,7 +131,7 @@ module.exports = function(src, filename, config) {
     output.renderFnStartLine,
     output.renderFnEndLine,
     templateLine
-  )
+  ).toJSON()
 
   return {
     code: output.code,