try to fix travis

.travis.yml

@@ -4,5 +4,6 @@ rvm:
   - 2.1
   - 2.2
   - 2.3
+before_install: gem install bundler -v 1.12.0
 cache: bundler
 sudo: false

mini_racer.gemspec

@@ -20,7 +20,7 @@ Gem::Specification.new do |spec|
   spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
   spec.require_paths = ["lib"]
 
-  #spec.add_development_dependency "bundler", "~> 1.12"
+  spec.add_development_dependency "bundler", "~> 1.12"
   spec.add_development_dependency "rake", "~> 10.0"
   spec.add_development_dependency "minitest", "~> 5.0"
   spec.add_development_dependency "rake-compiler"