Remove travis configuration (replaced with circle)

.travis.yml

@@ -1,26 +0,0 @@
-language: ruby
-
-sudo: false
-
-cache: bundler
-
-addons:
-  apt:
-    packages:
-    - bsdtar
-
-rvm:
-  - 2.3.8
-  - 2.4.5
-  - 2.5.3
-  - 2.6.1
-
-branches:
-  only:
-    - master
-
-env:
-  global:
-    - NOKOGIRI_USE_SYSTEM_LIBRARIES=true
-
-script: bundle exec rake test:unit