Drop support for node < 4

[mike-north]

CI is currently failing (even on master) due to many dependencies no longer working with node 0.10. Digging a bit deeper, there are several libraries that use both const and arrow functions.

Click for a list of npm WARNs received when building with node 0.10

``` npm WARN deprecated esperanto@0.6.34: Esperanto is deprecated in favour of http://rollupjs.org npm WARN engine boom@3.0.0: wanted: {"node":">=4.0.0"} (current: {"node":"0.10.48","npm":"2.15.1"}) npm WARN engine hoek@3.0.4: wanted: {"node":">=4.0.0"} (current: {"node":"0.10.48","npm":"2.15.1"}) npm WARN engine request@2.83.0: wanted: {"node":">= 4"} (current: {"node":"0.10.48","npm":"2.15.1"}) npm WARN engine form-data@2.3.1: wanted: {"node":">= 0.12"} (current: {"node":"0.10.48","npm":"2.15.1"}) npm WARN engine har-validator@5.0.3: wanted: {"node":">=4"} (current: {"node":"0.10.48","npm":"2.15.1"}) npm WARN engine hawk@6.0.2: wanted: {"node":">=4.5.0"} (current: {"node":"0.10.48","npm":"2.15.1"}) npm WARN engine har-schema@2.0.0: wanted: {"node":">=4"} (current: {"node":"0.10.48","npm":"2.15.1"}) npm WARN engine mime@1.6.0: wanted: {"node":">=4"} (current: {"node":"0.10.48","npm":"2.15.1"}) npm WARN engine boom@4.3.1: wanted: {"node":">=4.0.0"} (current: {"node":"0.10.48","npm":"2.15.1"}) npm WARN engine sntp@2.1.0: wanted: {"node":">=4.0.0"} (current: {"node":"0.10.48","npm":"2.15.1"}) npm WARN engine hoek@4.2.0: wanted: {"node":">=4.0.0"} (current: {"node":"0.10.48","npm":"2.15.1"}) npm WARN engine cryptiles@3.1.2: wanted: {"node":">=4.0.0"} (current: {"node":"0.10.48","npm":"2.15.1"}) npm WARN engine boom@5.2.0: wanted: {"node":">=4.0.0"} (current: {"node":"0.10.48","npm":"2.15.1"}) npm WARN engine co@4.6.0: wanted: {"iojs":">= 1.0.0","node":">= 0.12.0"} (current: {"node":"0.10.48","npm":"2.15.1"}) ```

[rwjblue]

Can you also update package.json with an engines entry?