Freeze inspec version for salt-formulas tests

- in inspec >= 3.0.0 there are changes in schema
	  which require to have tests/integration dir
	  to contain tests, otherwise fails
	- this is temporary workaround until we decide
	  proper fix

Change-Id: I6a1153afa5a8a7f9799347d2f2f3f1fd3d570b8f

.travis.yml

     gem 'test-kitchen'
     gem 'kitchen-docker'
     gem 'kitchen-inspec'
-    gem 'inspec'
+    gem 'inspec', '<3.0.0'
+    #Version was frozen, because of issues in the version of inspec >3.0.0 -- see https://mirantis.jira.com/browse/PROD-24324 for more info
     gem 'kitchen-salt', :git => 'https://github.com/salt-formulas/kitchen-salt.git'
   - bundle install