Allow emulator work work on node version ^6

[ThePaulMcBride]

Fixes #267

• [x] Tests and linter pass
• [x] Code coverage does not decrease (if any source code was changed)
• [x] Appropriate docs were updated (if necessary)

The restriction on the node version seems unnecessary. This causes issues with the firebase tool packages which relies on this.

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[ThePaulMcBride]

I signed it!

[googlebot]

CLAs look good, thanks!

[ThePaulMcBride]

Can I get some feedback here? This is an issues that is causing a lot of confusion to Firebase users.

[jmdobry]

Superseded by #287