Fix ruby version check in plugin test

Before
$ if ["${ruby}" = '2.7']; then echo yay; fi
bash: [2.7: command not found

After
$ if [ "${ruby}" = '2.7' ]; then echo yay; fi
yay

Fixes: f164a3f

theforeman.org/scripts/test/test_plugin.sh

@@ -23,7 +23,7 @@ rvm gemset empty --force
 
 set -x
 
-if ["${ruby}" = '2.7']
+if [ "${ruby}" = '2.7' ]
 then
     gem install bundler -v 2.4.22 --no-document
 else