Fix 'Your Gemfile.lock is corrupt.' on Travis CI

couple of things changed due to rubygems/bundler#4467

incl. Ruby (RVM) bump from 2.2.5 to 2.3.1 (just to be in line with local)

.gitignore

@@ -1,3 +1,8 @@
 _site/
 .sass-cache/
 .jekyll-metadata
+.bundle/
+vendor/
+
+# https://github.com/bundler/bundler/issues/4467
+Gemfile.lock

.travis.yml

@@ -5,7 +5,9 @@ sudo: false # route your build to the container-based infrastructure for a faste
 language: ruby
 
 rvm:
-- 2.2.5
+- 2.3.1
+
+install: bundle install
 
 script: ./.travis.sh
 

README.md

@@ -14,6 +14,21 @@ Your contributions are more than welcome!
 
 ## Troubleshooting
 
+Use `rm -rf .bundle/ vendor/ ; git clean -fX` to solve this problem if it occurs locally:
+
+```
+Your Gemfile.lock is corrupt. The following gem is missing from the DEPENDENCIES section: 'mini_portile2'
+
+```
+
+and if it occurs on Tracis CI then add `install: bundle install` to `.travis.yml`
+and git rm Gemfile.lock && echo "Gemfile.lock" >>.gitignore;
+needed because its default is `bundle install --jobs=3 --retry=3 --deployment`,
+the `--deployment` if a `Gemfile.lock` exists,
+see https://docs.travis-ci.com/user/languages/ruby/#Travis-CI-uses-Bundler,
+and that is causing some issues, see https://github.com/bundler/bundler/issues/4467.
+
+
 Use `sudo dnf install ruby-devel` to solve this problem:
 
 ```