Update yajl-ruby to 1.3.1

There is a security vulnerability in yajl-ruby 1.3.0:

In the yajl-ruby gem 1.3.0 for Ruby, when a crafted JSON file is
supplied to `Yajl::Parser.new.parse`, the whole ruby process crashes
with a `SIGABRT` in the `yajl_string_decode` function in
`yajl_encode.c`. This results in the whole ruby process terminating and
potentially a denial of service.

Reference:

https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2017-16516

Gemfile.lock

@@ -555,7 +555,7 @@ GEM
       chronic (>= 0.6.3)
     xpath (2.0.0)
       nokogiri (~> 1.3)
-    yajl-ruby (1.3.0)
+    yajl-ruby (1.3.1)
 
 PLATFORMS
   ruby
@@ -668,4 +668,4 @@ DEPENDENCIES
   whenever
 
 BUNDLED WITH
-   1.15.4
+   1.16.0