Upgrade gem dependencies

Gemfile

@@ -1,8 +1,13 @@
+# encoding: utf-8
+
 source 'https://rubygems.org'
 
 gemspec
 
-gem 'mutant',   path: '.'
+gem 'mutant', path: '.'
 
-gem 'devtools', git: 'https://github.com/rom-rb/devtools.git'
-eval(File.read(File.join(File.dirname(__FILE__), 'Gemfile.devtools')))
+group :development, :test do
+  gem 'devtools', git: 'https://github.com/rom-rb/devtools.git'
+end
+
+eval_gemfile File.join(File.dirname(__FILE__), 'Gemfile.devtools')

Gemfile.devtools

@@ -3,7 +3,7 @@
 group :development do
   gem 'rake',  '~> 10.1.0'
   gem 'rspec', '~> 2.14.1'
-  gem 'yard',  '~> 0.8.6.2'
+  gem 'yard',  '~> 0.8.7'
 end
 
 group :yard do
@@ -31,7 +31,7 @@ end
 
 group :metrics do
   gem 'coveralls', '~> 0.6.7'
-  gem 'flay',      '~> 2.3.1'
+  gem 'flay',      '~> 2.4.0'
   gem 'flog',      '~> 4.1.1'
   gem 'reek',      '~> 1.3.1',  git: 'https://github.com/troessner/reek.git'
   gem 'rubocop',   '~> 0.10.0', git: 'https://github.com/bbatsov/rubocop.git'
@@ -41,10 +41,6 @@ group :metrics do
   platforms :ruby_19, :ruby_20 do
     gem 'yard-spellcheck', '~> 0.1.5'
   end
-
-  platforms :rbx do
-    gem 'pelusa', '~> 0.2.2'
-  end
 end
 
 group :benchmarks do