Update dependencies and gems.

Also, remove postcss-zindex plugin and update browserslist config.

Author: XhmikosR

.browserslistrc

@@ -1,8 +1,7 @@
 # https://github.com/ai/browserslist#browserslist
 
-last 2 versions
+last 2 major versions
 > 1%
 Edge >= 12
 Explorer >= 9
 Firefox ESR
-Opera 12.1

Gemfile.lock

@@ -1,13 +1,13 @@
 GEM
   remote: https://rubygems.org/
   specs:
-    addressable (2.5.1)
-      public_suffix (~> 2.0, >= 2.0.2)
+    addressable (2.5.2)
+      public_suffix (>= 2.0.2, < 4.0)
     colorator (1.1.0)
     ffi (1.9.18)
     ffi (1.9.18-x64-mingw32)
     forwardable-extended (2.6.0)
-    jekyll (3.5.1)
+    jekyll (3.5.2)
       addressable (~> 2.4)
       colorator (~> 1.0)
       jekyll-sass-converter (~> 1.0)
@@ -30,15 +30,15 @@ GEM
       jekyll (~> 3.3)
     jekyll-watch (1.5.0)
       listen (~> 3.0, < 3.1)
-    kramdown (1.14.0)
+    kramdown (1.15.0)
     liquid (4.0.0)
     listen (3.0.8)
       rb-fsevent (~> 0.9, >= 0.9.4)
       rb-inotify (~> 0.9, >= 0.9.7)
     mercenary (0.3.6)
     pathutil (0.14.0)
       forwardable-extended (~> 2.6)
-    public_suffix (2.0.5)
+    public_suffix (3.0.0)
     rb-fsevent (0.10.2)
     rb-inotify (0.9.10)
       ffi (>= 0.5.0, < 2)
@@ -63,4 +63,4 @@ DEPENDENCIES
   jekyll-sitemap (~> 1.1.1)
 
 BUNDLED WITH
-   1.15.2
+   1.15.4

Gruntfile.js

@@ -97,7 +97,6 @@ module.exports = function(grunt) {
             options: {
                 processors: [
                     require('css-mqpacker')(), // combine media queries
-                    require('postcss-zindex')(), // reduce z-index values
                     require('postcss-combine-duplicated-selectors')({ removeDuplicatedProperties: true }), // combine duplicate selectors
                     require('autoprefixer')() // add vendor prefixes
                 ]