Add version numbers to all dependencies.

Reference all the latest versions available currently.

This will avoid breaking of JSDuck when one of the dependencies
has been upgraded to become imcompatible.  But it also introduces
a responsibility for us to update these version numbers whenever
a new JSDuck gem is released.

jsduck.gemspec

@@ -20,11 +20,11 @@ Gem::Specification.new do |s|
 
   s.executables = ["jsduck"]
 
-  s.add_dependency 'rdiscount'
-  s.add_dependency 'json'
-  s.add_dependency 'parallel'
+  s.add_dependency 'rdiscount', '~> 2.1.6'
+  s.add_dependency 'json', '~> 1.8.0'
+  s.add_dependency 'parallel', '~> 0.7.1'
   s.add_dependency 'rkelly-remix', '~> 0.0.2'
-  s.add_dependency 'dimensions'
+  s.add_dependency 'dimensions', '~> 1.2.0'
 
   s.add_development_dependency 'rspec'
   s.add_development_dependency 'rake'