Add mdl & awesome_bot as bundler deps

Also use .ruby-version and check using mdl in CI

.ruby-version

@@ -0,0 +1 @@
+2.4.2

.travis.yml

@@ -1,10 +1,5 @@
 language: ruby
 
-rvm:
-  - 2.2
-
-before_script:
-  - gem install awesome_bot
-
 script:
+  - mdl README.md
   - awesome_bot README.md --allow-redirect

Gemfile

@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+
+source "https://rubygems.org"
+
+git_source(:github) {|repo_name| "https://github.com/#{repo_name}" }
+
+gem "mdl"
+gem "awesome_bot"

Gemfile.lock

@@ -0,0 +1,25 @@
+GEM
+  remote: https://rubygems.org/
+  specs:
+    awesome_bot (1.18.0)
+      parallel (= 1.12.0)
+    kramdown (1.17.0)
+    mdl (0.5.0)
+      kramdown (~> 1.12, >= 1.12.0)
+      mixlib-cli (~> 1.7, >= 1.7.0)
+      mixlib-config (~> 2.2, >= 2.2.1)
+    mixlib-cli (1.7.0)
+    mixlib-config (2.2.13)
+      tomlrb
+    parallel (1.12.0)
+    tomlrb (1.2.7)
+
+PLATFORMS
+  ruby
+
+DEPENDENCIES
+  awesome_bot
+  mdl
+
+BUNDLED WITH
+   1.16.3