Bump bootstrap from 4.1.3 to 4.3.1

Bumps [bootstrap](https://github.com/twbs/bootstrap-rubygem) from 4.1.3 to 4.3.1.
- [Release notes](https://github.com/twbs/bootstrap-rubygem/releases)
- [Changelog](https://github.com/twbs/bootstrap-rubygem/blob/master/CHANGELOG.md)
- [Commits](twbs/bootstrap-rubygem@v4.1.3...v4.3.1)

Signed-off-by: dependabot[bot] <support@github.com>

Gemfile

@@ -9,7 +9,7 @@ gem 'uglifier', '>= 1.0.3'
 # gem 'therubyracer', platforms: :ruby
 # gem 'duktape'
 
-gem 'bootstrap', '~> 4.1.0'
+gem 'bootstrap', '~> 4.3.1'
 gem 'chart-js-rails'
 gem 'jquery-rails'
 gem 'jquery-ui-rails'

Gemfile.lock

@@ -64,10 +64,10 @@ GEM
     ast (2.4.1)
     autoprefixer-rails (10.0.1.0)
       execjs
-    bootstrap (4.1.3)
-      autoprefixer-rails (>= 6.0.3)
-      popper_js (>= 1.12.9, < 2)
-      sass (>= 3.5.2)
+    bootstrap (4.3.1)
+      autoprefixer-rails (>= 9.1.0)
+      popper_js (>= 1.14.3, < 2)
+      sassc-rails (>= 2.0.0)
     brakeman (4.10.0)
     builder (3.2.4)
     capistrano (3.14.1)
@@ -265,6 +265,14 @@ GEM
       sprockets (>= 2.8, < 4.0)
       sprockets-rails (>= 2.0, < 4.0)
       tilt (>= 1.1, < 3)
+    sassc (2.4.0)
+      ffi (~> 1.9)
+    sassc-rails (2.1.2)
+      railties (>= 4.0.0)
+      sassc (>= 2.0)
+      sprockets (> 3.0)
+      sprockets-rails
+      tilt
     select2-rails (4.0.13)
     sexp_processor (4.15.1)
     spawnling (2.1.5)
@@ -306,7 +314,7 @@ PLATFORMS
   ruby
 
 DEPENDENCIES
-  bootstrap (~> 4.1.0)
+  bootstrap (~> 4.3.1)
   brakeman
   capistrano (~> 3.11)
   capistrano-bundler