chore(versions): upgrade dependency-tree to 6.0.0

[akkumar]

Address the following warning.

npm WARN [email protected] requires a peer of typescript@~2.0.3 but none is installed. You must install peer dependencies yourself.

by upgrading dependency-tree to 6.0.0 (madge depends on the same. Independently, pr logged with madge as well to avoid explicitly upgrading dependency-tree - pahen/madge#147 ).

[CaerusKaru]

Since you have a PR open for madge, I see no reason to add an extra dev dependency here, even temporarily. It's only a warning, after all.

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