Lower npm engine required

[joshuaalpuerto]

• Updated package.json to work with node engine >=7
• Lower version of eslint from 4 back to 3 due to issue of yarn lint. Failed to compile after install eslint locally (propTypes is not defined) facebook/create-react-app#2528

[codecov-io]

Codecov Report

Merging #110 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #110   +/-   ##
=======================================
  Coverage   40.97%   40.97%           
=======================================
  Files           6        6           
  Lines         205      205           
=======================================
  Hits           84       84           
  Misses        121      121

Impacted Files	Coverage Δ
src/defaultProps.js
src/SelectionsPresenter.js
src/SuperSelectField.js
src/FloatingLabel.js
src/utils.js
src/types/index.js
...an/material-ui-superselectfield/src/types/index.js	33.33% <0%> (ø)
...ial-ui-superselectfield/src/SelectionsPresenter.js	100% <0%> (ø)
...n/material-ui-superselectfield/src/defaultProps.js	53.84% <0%> (ø)
...terial-ui-superselectfield/src/SuperSelectField.js	38.84% <0%> (ø)
... and 2 more

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