Explicitly set encoding for Precious::App

An issue was reported (see #1721) where, in docker containers where the
`LANG` was not being set, `Precious::App` would serve Mustache templates
in an ASCII encoding. This caused templates to error out.

In the past, this would have happened in environments where `LANG` was
not set and the Gollum applciation configuration had enabled MathJax.
Now, it happens even if MathJax is disabled because of a UTF-8 character
I added to the mobile navigation menu.

Basically, we should enforce a UTF-8 encoding from now on to avoid
runtime errors related to ASCII encoding.

.travis.yml

@@ -2,7 +2,7 @@ rvm:
   - 2.4.0
   - 2.6.0
   - 3.0.0
-  - jruby-9.2.9.0
+  - jruby-9.2.18.0
 jdk:
   - oraclejdk9
 before_install:

lib/gollum/app.rb

@@ -1,4 +1,5 @@
-# ~*~ encoding: utf-8 ~*~
+# encoding: UTF-8
+
 require 'cgi'
 require 'sinatra'
 require 'sinatra/namespace'
@@ -69,6 +70,8 @@ module Precious
     register Sinatra::Namespace
     include Precious::Helpers
 
+    Encoding.default_external = "UTF-8"
+
     dir = File.dirname(File.expand_path(__FILE__))
 
     set :sprockets, ::Precious::Assets.sprockets(dir)