Set node version to v10.15.3

[tay1orjones]

Closes #735

Summary

• Set node version to v10.15.3 via .nvmrc

Why this version?

1. v10 is an Active LTS version through April 2020
2. v10.15.3 specifically is preinstalled on the VM used by Travis CI. By specifying this particular version, we get an added benefit of slightly reduced build times.
3. v8 is only Maintenance LTS
4. v12 has potential compatibility issues we have yet to test and resolve ([node] v12 support #736)

Change List (commits, features, bugs, etc)

• add .nvmrc
• add documentation for .nvmrc
• remove version from .travis.yml, it utilizes the .nvmrc

Acceptance Test (how to verify the PR)

• status checks should pass
• once merged, it will be prudent to closely watch the Travis CI build in master to ensure everything builds and deploys smoothly

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[Potato-W]

In my environment, when I use node v10.13.0, there is an error:
error brotli-size@4.0.0: The engine "node" is incompatible with this module. Expected version ">= 10.16.0". Got "10.13.0".
when I switch to v10.16.0, it works. So I think maybe 10.15.3 may be same error above.

[tay1orjones]

@ChangShunWang please try with 10.15.3. From my testing there are no errors for me in mac and windows WSL using 10.15.3 on this branch for install, build, and start.

I'm not sure why you're seeing that error - the only reference to brotli-size in our yarn.lock is for version 0.0.3, not v4:

https://github.com/IBM/carbon-addons-iot-react/blob/b5ab780ccd457b82bc32675874295efb4e48e621/yarn.lock#L5109-L5114

Locally, yarn reports the same when using v10.15.3

❯ yarn why brotli-size
yarn why v1.12.3
[1/4] 🤔  Why do we have the module "brotli-size"...?
[2/4] 🚚  Initialising dependency graph...
[3/4] 🔍  Finding dependency...
[4/4] 🚡  Calculating file sizes...
=> Found "brotli-size@0.0.3"
info Reasons this module exists
   - "rollup-plugin-filesize" depends on it
   - Hoisted from "rollup-plugin-filesize#brotli-size"
info Disk size without dependencies: "16KB"
info Disk size with unique dependencies: "3.11MB"
info Disk size with transitive dependencies: "5.63MB"
info Number of shared dependencies: 36
✨  Done in 1.07s.

[davidicus]

LGTM 👍

[tay1orjones]

🎉 This PR is included in version 2.17.2 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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