Upgrade statemachines gem

This prepares us for the upgrade to Rails 6

Gemfile

@@ -175,7 +175,7 @@ group :puma do
 end
 
 # State machine
-gem 'state_machines-activerecord', '~> 0.5.1'
+gem 'state_machines-activerecord', '~> 0.6.0'
 
 # Issue tags
 gem 'acts-as-taggable-on', '~> 6.0'

Gemfile.lock

@@ -976,11 +976,11 @@ GEM
     sshkey (2.0.0)
     stackprof (0.2.10)
     state_machines (0.5.0)
-    state_machines-activemodel (0.5.1)
-      activemodel (>= 4.1, < 6.0)
+    state_machines-activemodel (0.7.1)
+      activemodel (>= 4.1)
       state_machines (>= 0.5.0)
-    state_machines-activerecord (0.5.1)
-      activerecord (>= 4.1, < 6.0)
+    state_machines-activerecord (0.6.0)
+      activerecord (>= 4.1)
       state_machines-activemodel (>= 0.5.0)
     swd (1.1.2)
       activesupport (>= 3)
@@ -1310,7 +1310,7 @@ DEPENDENCIES
   sprockets (~> 3.7.0)
   sshkey (~> 2.0)
   stackprof (~> 0.2.10)
-  state_machines-activerecord (~> 0.5.1)
+  state_machines-activerecord (~> 0.6.0)
   sys-filesystem (~> 1.1.6)
   test-prof (~> 0.10.0)
   thin (~> 1.7.0)