Bump firrtl from `7dc1ad0` to `b904cd0` (#151)

Bumps [firrtl](https://github.com/freechipsproject/firrtl) from `7dc1ad0` to `b904cd0`.
- [Release notes](https://github.com/freechipsproject/firrtl/releases)
- [Commits](https://github.com/freechipsproject/firrtl/compare/7dc1ad04c04ebefa9cc393430bf9deff8a1ffa46...b904cd0c00f4f0d4fdfd9db90961aaf31aa191b3

)

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