Disable platform specific gem installs via Bundler

See discussion here: #292 (comment)

To ensure the setting is persisted for every user of the repository, I
decided to track the Bundler config file. This seems like a good idea
anyway, even though Bundler currently has a bug resulting in that file
being changed sometimes, which can be confusing for developers.

See rubygems/rubygems#3708

Author: mokagio

.bundle/config

@@ -0,0 +1,3 @@
+---
+BUNDLE_SPECIFIC_PLATFORM: "false"
+BUNDLE_WITH: "test"

.gitignore

@@ -5,7 +5,6 @@
 .idea
 
 # Ruby / Bundler
-.bundle/
 vendor/bundle
 
 # Fastlane

Gemfile.lock

@@ -260,6 +260,7 @@ GEM
     method_source (0.9.2)
     mini_magick (4.11.0)
     mini_mime (1.1.0)
+    mini_portile2 (2.5.3)
     minitest (5.14.4)
     molinillo (0.6.6)
     multi_json (1.15.0)
@@ -269,7 +270,8 @@ GEM
     naturally (2.2.1)
     netrc (0.11.0)
     no_proxy_fix (0.1.2)
-    nokogiri (1.11.7-x86_64-darwin)
+    nokogiri (1.11.7)
+      mini_portile2 (~> 2.5.0)
       racc (~> 1.4)
     octokit (4.21.0)
       faraday (>= 0.9)
@@ -415,4 +417,4 @@ DEPENDENCIES
   yard
 
 BUNDLED WITH
-   2.2.24
+   2.2.25