Add Node.js version 10 to .travis.yml

[karlhorky]

Closes #57

Originally was going to remove Node 6 too, but maybe let's wait for end of life in April 2019.

[karlhorky]

Needed to update package-lock.json as well to make CI run through on Node 10:

$ npm ci 
npm ERR! cipm can only install packages when your package.json and package-lock.json or npm-shrinkwrap.json are in sync. Please update your lock file with `npm install` before continuing.
npm ERR! 
npm ERR! 
npm ERR! Invalid: lock file's mocha@4.0.1 does not satisfy mocha@^5.0.0
npm ERR! Invalid: lock file's sinon@3.3.0 does not satisfy sinon@^4.1.2
npm ERR! 
npm ERR! A complete log of this run can be found in:
npm ERR!     /home/travis/.npm/_logs/2018-07-16T15_59_48_127Z-debug.log
The command "eval npm ci  " failed. Retrying, 2 of 3.
npm ERR! cipm can only install packages when your package.json and package-lock.json or npm-shrinkwrap.json are in sync. Please update your lock file with `npm install` before continuing.
npm ERR! 
npm ERR! 
npm ERR! Invalid: lock file's mocha@4.0.1 does not satisfy mocha@^5.0.0
npm ERR! Invalid: lock file's sinon@3.3.0 does not satisfy sinon@^4.1.2
npm ERR! 
npm ERR! A complete log of this run can be found in:
npm ERR!     /home/travis/.npm/_logs/2018-07-16T15_59_49_629Z-debug.log
The command "eval npm ci  " failed. Retrying, 3 of 3.
npm ERR! cipm can only install packages when your package.json and package-lock.json or npm-shrinkwrap.json are in sync. Please update your lock file with `npm install` before continuing.
npm ERR! 
npm ERR! 
npm ERR! Invalid: lock file's mocha@4.0.1 does not satisfy mocha@^5.0.0
npm ERR! Invalid: lock file's sinon@3.3.0 does not satisfy sinon@^4.1.2
npm ERR! 
npm ERR! A complete log of this run can be found in:
npm ERR!     /home/travis/.npm/_logs/2018-07-16T15_59_51_133Z-debug.log
The command "eval npm ci  " failed 3 times.
The command "npm ci " failed and exited with 1 during .