Upgrade PostCSS build deps (#681)

* Move supported browser list to package.json.

That way all build tools can use it.

* Specify supported browsers explicitly.

This is necessary for the latest version of browserslist/autoprefixer.

This also drops IE 10 support, as it was accidental anyway.

* Upgrade PostCSS build deps.

package.json

@@ -63,9 +63,19 @@
   ],
   "author": "Netlify",
   "license": "MIT",
+  "browserslist": [
+    "last 2 Chrome versions",
+    "last 2 Opera versions",
+    "last 2 Firefox versions",
+    "last 2 Edge versions",
+    "last 2 Safari versions",
+    "last 2 iOS versions",
+    "last 2 ChromeAndroid versions",
+    "IE 11",
+    "IE_mob 11"
+  ],
   "devDependencies": {
     "all-contributors-cli": "^4.4.0",
-    "autoprefixer": "^6.3.3",
     "babel": "^6.5.2",
     "babel-cli": "^6.18.0",
     "babel-core": "^6.23.1",
@@ -77,6 +87,7 @@
     "babel-preset-react": "^6.23.0",
     "babel-preset-stage-1": "^6.22.0",
     "babel-runtime": "^6.23.0",
+    "caniuse-lite": "^1.0.30000745",
     "cross-env": "^5.0.2",
     "css-loader": "^0.28.7",
     "deep-equal": "^1.0.1",
@@ -94,9 +105,9 @@
     "jest-cli": "^21.2.1",
     "lint-staged": "^3.3.1",
     "npm-check": "^5.2.3",
-    "postcss-cssnext": "^2.7.0",
-    "postcss-import": "^10.0.0",
-    "postcss-loader": "^2.0.5",
+    "postcss-cssnext": "^3.0.2",
+    "postcss-import": "^11.0.0",
+    "postcss-loader": "^2.0.7",
     "react-test-renderer": "^15.5.0",
     "style-loader": "^0.18.2",
     "stylefmt": "^4.3.1",