Merge #686: Bump Ruby, Jekyll and jQuery

c6b2817 Bump jQuery to 3.4.1 (Sjors Provoost)
9aceff7 Upgrade Ruby to 2.5.1 and Jekyll from  3.0.2 to 3.8.5 (Sjors Provoost)
7526e8a Add .ruby-version for rvm and rbenv (Sjors Provoost)

Pull request description:

  Builds on top of #684
  * Ruby 2.2.2 is end of life, so bumping to the latest and greatest 2.6.5
  * `bundle update` mainly:
    * bumps Jekyll from 3.0.2 to 3.8.5, which required small tweaks to two files, but otherwise seems to behave
    * bumps html-proofer from 2.6.4 to 3.14.1 which is a bit more strict, hence the link fixes
  * bumping jQuery from 1.9.1 to 3.4.1, from here: https://jquery.com/download/ (compressed, production jQuery 3.4.1), I didn't try the slim build

ACKs for top commit:
  harding:
    Tested ACK c6b2817  (reviewed code, skimmed very large diff of site build with this merged on master versus just master, verified hash of updated jquery matches [upstream](https://code.jquery.com/jquery-3.4.1.min.js))

Tree-SHA512: f036d27aaa6322150911a028ad7159c723a8010f5ea81eb7e7dfc5afec274cc64ef29a9752f7b3d63f875659c123dc9cbee82ff6ca7ad392e5c7805b3ba215af

Author: harding

.ruby-version

@@ -0,0 +1 @@
+2.5.1

.travis.yml

@@ -1,10 +1,12 @@
 language: ruby
-rvm:
-  - "2.2.2"
 
 ## Save bundler deps.
 cache: bundler
 
+before_install:
+  - gem update --system
+  - gem --version
+
 ## Disable external link checking to prevent spurious failures because
 ## of other people's downtime. This can also avoid wasting their
 ## bandwidth.
@@ -16,7 +18,7 @@ script:
 env:
   global:
     - NOKOGIRI_USE_SYSTEM_LIBRARIES=true # speeds up installation of html-proofer
-    
+
 # Whitelist all branches
 branches:
   only:

Gemfile

@@ -1,9 +1,10 @@
 source 'https://rubygems.org'
 
-## If you update the version here, also update it in .travis.yml and
-## README.md. Then push your branch and make sure Travis supports that
+## This version should match the version in Debian Stable.
+## If you update the version here, also update it in .ruby-version, .travis.yml
+## and README.md. Then push your branch and make sure Travis supports that
 ## version.
-ruby '2.2.2'
+ruby '2.5.1'
 
 ## If you add a new Gem below, run `bundle install` to install it.
 group :development do