Update dependency postcss-loader to v3

[renovate]

This PR contains the following updates:

Package	Type	Update	Change	References
postcss-loader	dependencies	major	2.1.6 -> 3.0.0	source

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Release Notes

postcss/postcss-loader

v3.0.0

Compare Source

Bug Fixes

• index: add ast version (meta.ast) (f34954f)
• index: emit warnings as an instance of {Error} (8ac6fb5)
• options: improved ValidationError messages (549ea08)

Chores

• package: update postcss v6.0.0...7.0.0 (dependencies) (#​375) (daa0da8)

BREAKING CHANGES

• package: requires node >= v6.0.0

2.1.6 (2018-07-10)

Bug Fixes

• package: config memory leak, updates postcss-load-config v1.2.0...2.0.0 (dependencies) (0547b12)

2.1.5 (2018-05-04)

Bug Fixes

• index: remove sourceMap warning (#​361) (4416791)

2.1.4 (2018-04-16)

Bug Fixes

• restore loader object in postcss config context (#​355) (2ff1735)

2.1.3 (2018-03-20)

Bug Fixes

• options: revert additionalProperties changes to keep SemVer (bd7fc38)

2.1.2 (2018-03-17)

Bug Fixes

• options: disallow additional properties and add ident to validation (#​346) (82ef553)

2.1.1 (2018-02-26)

Bug Fixes

• index: don't set to value (options) (#​339) (cdbb8b6)

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