Address warning about open-ended dependency on railties

When building the gem (`gem build external_asset_pipeline.gemspec`),
this was being logged:

```
WARNING:  open-ended dependency on railties (>= 5.0.0) is not recommended
```

Adding an upper bound on the version resolves the warning.

Author: rmacklin

examples/demo_app-no-sprockets/Gemfile.lock

@@ -2,7 +2,7 @@ PATH
   remote: ../..
   specs:
     external_asset_pipeline (0.1.0)
-      railties (>= 5.0.0)
+      railties (>= 5.0.0, < 7.0)
 
 GEM
   remote: https://rubygems.org/

examples/demo_app/Gemfile.lock

@@ -2,7 +2,7 @@ PATH
   remote: ../..
   specs:
     external_asset_pipeline (0.1.0)
-      railties (>= 5.0.0)
+      railties (>= 5.0.0, < 7.0)
 
 GEM
   remote: https://rubygems.org/

external_asset_pipeline.gemspec

@@ -30,5 +30,5 @@ Gem::Specification.new do |s|
   s.files = Dir['{lib}/**/*']
   s.require_paths = ['lib']
 
-  s.add_runtime_dependency 'railties', '>= 5.0.0'
+  s.add_runtime_dependency 'railties', ['>= 5.0.0', '< 7.0']
 end