update travis (#39)

* update travis

* require rake 12.x

* travis-ci/travis-ci#8978

Author: KazuCocoa

.travis.yml

@@ -4,7 +4,12 @@ language: ruby
 rvm:
   - 2.2.5
   - 2.3.3
-  - 2.4.0
+  - 2.4.3
+  - 2.5.0
+
+before_install:
+  - gem update --system
+  - gem update bundler
 
 script:
   - bundle exec rake rubocop

appium_lib_core.gemspec

@@ -26,7 +26,7 @@ Gem::Specification.new do |spec|
   spec.add_runtime_dependency 'json', '>= 1.8'
 
   spec.add_development_dependency 'bundler', '~> 1.14'
-  spec.add_development_dependency 'rake', '~> 10.0'
+  spec.add_development_dependency 'rake', '~> 12.0'
   spec.add_development_dependency 'yard', '~> 0.9.11'
   spec.add_development_dependency 'minitest', '~> 5.0'
   spec.add_development_dependency 'minitest-reporters', '~> 1.1'