Upgrade rubocop to fix Style/WordArray warning.

A few users have run into this problem, and upgrading rubocop
(specifically the parsing dependencies) fixes the issue. Example engine
output is shown below.

Before this change
==================

```
An error occurred while Style/WordArray cop was inspecting
config/initializers/logs.rb.
To see the complete backtrace run rubocop -d.
```

After this change
=================

```
warning: parser/current is loading parser/ruby22, which recognizes
warning: 2.2.3-compliant syntax, but you are running 2.2.2.
warning: please see
https://github.com/whitequark/parser#compatibility-with-ruby-mri.
```

Note: this version warning is new, but as there were no syntax changes
introduced in ruby 2.2.3, I don't believe it is an issue.
(https://www.ruby-lang.org/en/news/2015/08/18/ruby-2-2-3-released/)

Gemfile.lock

@@ -8,7 +8,7 @@ GEM
       thread_safe (~> 0.3, >= 0.3.4)
       tzinfo (~> 1.1)
     ansi (1.5.0)
-    ast (2.1.0)
+    ast (2.2.0)
     astrolabe (1.3.1)
       parser (~> 2.2)
     builder (3.2.2)
@@ -25,7 +25,7 @@ GEM
       ruby-progressbar
     mocha (1.1.0)
       metaclass (~> 0.0.1)
-    parser (2.2.2.6)
+    parser (2.2.3.0)
       ast (>= 1.1, < 3.0)
     powerpack (0.1.1)
     pry (0.10.1)
@@ -34,17 +34,19 @@ GEM
       slop (~> 3.4)
     rainbow (2.0.0)
     rake (10.4.2)
-    rubocop (0.33.0)
+    rubocop (0.35.1)
       astrolabe (~> 1.3)
-      parser (>= 2.2.2.5, < 3.0)
+      parser (>= 2.2.3.0, < 3.0)
       powerpack (~> 0.1)
       rainbow (>= 1.99.1, < 3.0)
-      ruby-progressbar (~> 1.4)
+      ruby-progressbar (~> 1.7)
+      tins (<= 1.6.0)
     rubocop-rspec (1.3.0)
     ruby-progressbar (1.7.5)
     safe_yaml (1.0.4)
     slop (3.6.0)
     thread_safe (0.3.5)
+    tins (1.6.0)
     tzinfo (1.2.2)
       thread_safe (~> 0.1)