github.com/BurntSushi/toml dependency out of sync with upstream

It looks like we're currently vendoring a version of this dependency that either never existed, or has since been written out of upstream's history (ffbe061dbf09df6f4d404ef06736cc2b9e2510f7 / v0.1.0-24-gffbe061).

Propose we update to the v0.2.0 tag (tests fail on v0.1.0 due to missing support for omitempty).