fix(package): downgrade `source-map` v0.7.0...0.5.6

[hackeryarn]

Please check if the PR fulfills these requirements

• [x] Tests for the changes have been added (for bug fixes / features)
• [x] Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

• [x] Bugfix
• [ ] Feature
• [ ] Code style update (formatting, local variables)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [ ] CI related changes
• [ ] Other... Please describe:

What is the current behavior? (You can also link to an open issue here) Install breaks on node < 8

What is the new behavior? Install is possible on node < 8

Does this PR introduce a breaking change?

• [ ] Yes
• [x] No

Other information: Resolves https://github.com/webpack-contrib/karma-webpack/issues/308

[jsf-clabot]

All committers have signed the CLA.

[michael-ciniawsky]

Released in v2.0.12 🎉