Relates to graphql#425.

- Swapped deprecated babel-preset-es2015 for babel-preset-env.
- Added package.json engines field.
- Configured babel-preset-env to target the Node.js version defined in package.json engines field.
- Kept Babel at v6, but moved config into a v7 ready .babelrc.js file for dynamic config via env.
- Tidied the package.json scripts, added an ESM build step that generates .mjs dist files. Follow the lead of graphql/graphql-js#1244, although I do things quite different for my own projects with Babel v7.
- Updated package.json main field for ESM/CJS cross compatibility.
- Added a package.json module field for tree-shaking bundlers.

This one needs careful consideration.

This is a nesesary breaking change. This is consistent with the graphql package, which only has named exports.

…nto mjs

# Conflicts:
#	src/__tests__/usage-test.js

- Configured Travis to test on the latest stable Node.js version (v9-10+ were not being tested!), and the lowest supported version which is now v6. Removed unessesary versions in between.
- Updated package.json engines field, which in turn updates transpilation via dynamic Babel preset-env config.