Pin Node version to 6.10.0

[aaronschachter]

What's this PR do?

Pins Node version to 6.10. We were previously always using the latest version of Node 🤦‍♀️

How should this be reviewed?

Deploy to heroku and confirm the build logs look like this:

-----> Installing binaries
       engines.node (package.json):  6.10.x
       engines.npm (package.json):   unspecified (use default)
       
       Resolving node version 6.10.x via semver.io...
       Downloading and installing node 6.10.0...
       Using default npm version: 3.10.10
-----> Restoring cache

Instead of this:

-----> Installing binaries
       engines.node (package.json):  >=6.10.x
       engines.npm (package.json):   unspecified (use default)
       
       Resolving node version >=6.10.x via semver.io...
       Downloading and installing node 7.7.2...

Any background context you want to provide?

This should close out #811 if Node 7.7.2 is the culprit as suggested in https://github.com/DoSomething/gambit/issues/811#issuecomment-285753435

Relevant tickets

Fixes #811

Checklist

• Tested on staging.

[sergiitk]

👍