Updated readme + .travis.ci

• Removed istanbul from the readme as default install step
• Dropped old version of node from the TravisCI manifest

ES2016

• ES2016 support (thanks @mikeal)
• Switched to istanbul-lib-instrument
• Updated development dependencies

Lazy-required `istanbul`

• Only require istanbul when options.instrumenter is not set. #34 #33
• Updated dependencies
• Housecleaning