Set NODE_ENV env var to 'production' when building

[KyleAMathews]

Fixes #337

There seems to be a few Webpack loaders that require this e.g.
sass-loader. We set NODE_ENV to production already inside Webpack e.g.
for React's optimizations but this PR is necessary for code outside
Webpack e.g. node code in Webpack loaders.

[oliverbenns]

Would love to be able to set my own NODE_ENV, e.g. NODE_ENV=version1 gatsby build.

[KyleAMathews]

@oliverbenns you generally don't want to override NODE_ENV as it has specific meaning e.g. many modules including React itself look at NODE_ENV to see if it is set to production to see if they should apply optimizations e.g. turning off dev checks. If you override that, your site will become significantly slower.

Want you want to do probably is use a different environment variable e.g. BUILD_VERSION=1 and then use the same technique as Gatsby core uses to make that env variable available to your frontend code

gatsby/lib/utils/webpack.config.js

Line 125 in 6c49ad9

'process.env': {