bump ruby version to 2.4.2

Versions older than 2.4.x don't build with recent libssl
(because of all of the deprecated bits). Since we don't have
a strong preference on our ruby version, except to remain
reasonably up-to-date, let's just jump right into the 2.4.x
series.

.ruby-version

@@ -1 +1 @@
-2.2.7
+2.4.2

.travis.yml

@@ -1,7 +1,7 @@
 language: ruby
 script: 'script/travis-ci'
 rvm:
-  - 2.2.7
+  - 2.4.2
 notifications:
   email: false
 

Gemfile

@@ -1,5 +1,5 @@
 source 'https://rubygems.org'
-ruby "2.2.7"
+ruby "2.4.2"
 
 gem 'rails', '4.2.6'
 gem 'rails_12factor', group: :production

Gemfile.lock

@@ -304,7 +304,7 @@ DEPENDENCIES
   yajl-ruby
 
 RUBY VERSION
-   ruby 2.2.7p470
+   ruby 2.4.2p198
 
 BUNDLED WITH
    1.15.4