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

@@ -12,7 +12,7 @@ gem 'uglifier', '>= 1.0.3'
 
 gem 'jquery-ui-rails'
 gem 'jquery-rails'
-gem 'bootstrap', '~> 4.1.0'
+gem 'bootstrap', '~> 4.3.1'
 gem 'chart-js-rails'
 gem 'select2-rails'
 

Gemfile.lock

@@ -66,10 +66,10 @@ GEM
     arel (7.1.4)
     autoprefixer-rails (9.7.0)
       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)
     builder (3.2.3)
     capistrano (3.8.2)
       airbrussh (>= 1.0.0)
@@ -214,6 +214,14 @@ GEM
       sprockets (>= 2.8, < 4.0)
       sprockets-rails (>= 2.0, < 4.0)
       tilt (>= 1.1, < 3)
+    sassc (2.2.1)
+      ffi (~> 1.9)
+    sassc-rails (2.1.2)
+      railties (>= 4.0.0)
+      sassc (>= 2.0)
+      sprockets (> 3.0)
+      sprockets-rails
+      tilt
     select2-rails (4.0.3)
       thor (~> 0.14)
     sexp_processor (4.13.0)
@@ -253,7 +261,7 @@ PLATFORMS
   x86_64-darwin-17
 
 DEPENDENCIES
-  bootstrap (~> 4.1.0)
+  bootstrap (~> 4.3.1)
   capistrano (~> 3.8.2)
   capistrano-bundler
   capistrano-locally