Remove node 6 and webpack 2 support

[chrisblossom]

Since we are going to have to do a major version for #134 and #138 I think it is a good idea to remove node 6 support as well as webpack 2.

[codecov-io]

Codecov Report

Merging #142 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #142   +/-   ##
=======================================
  Coverage   98.71%   98.71%           
=======================================
  Files           1        1           
  Lines          78       78           
  Branches       27       27           
=======================================
  Hits           77       77           
  Misses          1        1

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