Issue 58: Bump ruby version to 2.4.2

Closes #58. 2.4.2 is the latest version of ruby [supported by Heroku](https://devcenter.heroku.com/articles/ruby-support#supported-runtimes). The current version is causing deploys of this app to Heroku to fail.

This also updates the `json` version in the lockfile [known issue](ruby/json#286) causing it to fail to build locally or on Heroku.

Author: ebmshenfield

.travis.yml

@@ -1,5 +1,5 @@
 language: ruby
 rvm:
-  - ruby-2.1.2
+  - ruby-2.4.2
 notifications:
   webhooks: http://project-monitor.codeforamerica.org/projects/d702edae-5a33-478b-9d08-acc4a7826533/status

Gemfile

@@ -1,6 +1,6 @@
 source 'https://rubygems.org'
 
-ruby '2.1.2'
+ruby '2.4.2'
 
 gem 'twitter'
 gem 'dotenv'

Gemfile.lock

@@ -15,7 +15,7 @@ GEM
     http (0.6.2)
       http_parser.rb (~> 0.6.0)
     http_parser.rb (0.6.0)
-    json (1.8.1)
+    json (1.8.6)
     memoizable (0.4.2)
       thread_safe (~> 0.3, >= 0.3.1)
     minitest (5.4.1)
@@ -52,3 +52,9 @@ DEPENDENCIES
   twitter
   vcr
   webmock
+
+RUBY VERSION
+   ruby 2.4.2p198
+
+BUNDLED WITH
+   1.15.4