Downgrade bundler version requirement.

Previous is what gem scaffolding set, but Travis doesn't seem
to like it. Current is what I found in jekyll-feed, which is
where I modeled the Travis set up from.

Author: mpchadwick

jekyll-pre-commit.gemspec

@@ -19,7 +19,7 @@ Gem::Specification.new do |spec|
   spec.require_paths = ["lib"]
 
   spec.add_development_dependency "jekyll", ">= 3.3.0"
-  spec.add_development_dependency "bundler", "~> 1.10"
+  spec.add_development_dependency "bundler", "~> 1.6"
   spec.add_development_dependency "rake", "~> 10.0"
   spec.add_development_dependency "rspec", "~> 3.0"
 end