Update webpack to version 2.3.x

[cheremukhin23]

Updates webpack version to ^2.3.1 and changes its output path to absolute.
Fixes webpack error:

• configuration.output.path: The provided value "../app/assets/webpack" is not an absolute path!
  (Webpack 2.3 is breaking builds #770)

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This change is 

[coveralls]

Changes Unknown when pulling 26e4a04 on cheremukhin23:update-to-webpack-v2.3.1 into ** on shakacode:master**.

[justin808]

Thanks @cheremukhin23!