Compiler in watch mode does not track changes in .graphql file

For better dev experience will be cool if relay compiler recompile all files if catches changes in `schema.graphql` file.

Expected behavior:
1. Start compiler in watch mode `relay-compiler --src ./src --schema ./schema.graphql`
2. Change something in `schema.graphql` file
3. Should be called `CodegenRunner.compileAll` automatically without restarting `relay-compiler`

cc @kassens