Fix: Resupport version 2.2.9

Version is actually still supported

Revert "Drop unsupported version from CI"

This reverts commit 93dd1e132ffcd85ce323e32bbc463583103b7ad9.

Fix 2.2.9 incompatibility

This creates a performance hit but as this code is only test code, I deem this acceptable

.travis.yml

 language: ruby
 sudo: false
 rvm:
+  - 2.2.9
   - 2.3.6
   - 2.4.3
   - 2.5.0

spec/lib/guard/dsl_describer_spec.rb

@@ -29,7 +29,10 @@ RSpec.describe Guard::DslDescriber do
     stub_const 'Guard::Test', class_double('Guard::Plugin')
     stub_const 'Guard::Another', class_double('Guard::Plugin')
-    @output = +''
+    # TODO: Reenable rubocop and refactor to +'' one 2.2 compatibility is dropped
+    # rubocop:disable Performance/UnfreezeString - +'' is incompatible with MRI 2.2.9
+    @output = String.new
+    # rubocop:enable Performance/UnfreezeString
     # Strip escape sequences
     allow(STDOUT).to receive(:tty?).and_return(false)