move rack from runtime to dev dependency

Gemfile.lock

@@ -2,7 +2,7 @@ PATH
   remote: .
   specs:
     zero (0.0.1)
-      rack
+      tilt
 
 GEM
   remote: https://rubygems.org/
@@ -38,6 +38,7 @@ GEM
     rspec-mocks (2.12.0)
     slop (3.3.3)
     thor (0.16.0)
+    tilt (1.3.3)
 
 PLATFORMS
   ruby
@@ -46,6 +47,7 @@ DEPENDENCIES
   guard
   guard-bundler
   guard-rspec
+  rack
   rspec
   thor
   zero!

zero.gemspec

@@ -17,8 +17,9 @@ Gem::Specification.new do |s|
 
   s.rubygems_version = '1.8.24'
 
-  s.add_dependency('rack')
+  s.add_dependency('tilt')
 
   s.add_development_dependency('thor')
+  s.add_development_dependency('rack')
   s.add_development_dependency('rspec')
 end