Recommend Ruby 2.2, drop Ruby 2.0 support

As per https://www.loomio.org/d/VuwvXWU7/ruby-versions?proposal=DM8zidhc

.ruby-version

@@ -1 +1 @@
-2.1
+2.2

.travis.yml

@@ -1,8 +1,8 @@
 language: ruby
 
 rvm:
+  - 2.2
   - 2.1
-  - 2.0
 
 env:
   - DB=postgres BUILD_TYPE=other

Changelog.md

@@ -1,5 +1,10 @@
 # 0.6.0.0
 
+## Supported Ruby versions
+
+This release recommends using Ruby 2.2, while retaining Ruby 2.1 as an officially supported version.
+Ruby 2.0 is no longer officially supported.
+
 ## Refactor
 
 ## Bug fixes