Update Ruby to version 2.4.4 (mastodon#6964)

https://www.ruby-lang.org/en/news/2018/03/28/ruby-2-4-4-released/

> This release includes some bug fixes and some security fixes.
>
> - CVE-2017-17742: HTTP response splitting in WEBrick
> - CVE-2018-6914: Unintentional file and directory creation with directory traversal in tempfile and tmpdir
> - CVE-2018-8777: DoS by large request in WEBrick
> - CVE-2018-8778: Buffer under-read in String#unpack
> - CVE-2018-8779: Unintentional socket creation by poisoned NUL byte in UNIXServer and UNIXSocket
> - CVE-2018-8780: Unintentional directory traversal by poisoned NUL byte in Dir
> - Multiple vulnerabilities in RubyGems

.ruby-version

@@ -1 +1 @@
-2.5.0
+2.5.1

.travis.yml

@@ -39,7 +39,7 @@ addons:
       - yarn
 
 rvm:
-  - 2.4.2
+  - 2.4.3
   - 2.5.0
 
 services:

Dockerfile

@@ -1,4 +1,4 @@
-FROM ruby:2.4.3-alpine3.6
+FROM ruby:2.4.4-alpine3.6
 
 LABEL maintainer="https://github.com/tootsuite/mastodon" \
       description="Your self-hosted, globally interconnected microblogging community"