Test both rest-client 1.8 and latest

Until we can require 2.0 - next major release?

In theory our gemspec lets you combine any ancient
rest-client with kubeclient but not really,
our tests fail on 1.7.

.Gemfile-rest-client-1.8.rb

@@ -0,0 +1,11 @@
+# Allow travis to test rest-client 1.x.
+
+source 'https://rubygems.org'
+
+# Specify your gem's dependencies in kubeclient.gemspec
+gemspec
+
+if dependencies.any? # needed for overriding with recent bundler (1.13 ?)
+  dependencies.delete('rest-client')
+  gem 'rest-client', "= 1.8.0"
+end

.travis.yml

@@ -4,9 +4,17 @@ rvm:
   - "2.1"
   - "2.2"
   - "2.3.0"
+gemfile:
+  - Gemfile
+  - .Gemfile-rest-client-1.8.rb
 sudo: false
 cache: bundler
 script: bundle exec rake $TASK
 env:
  - TASK=test
  - TASK=rubocop
+matrix:
+  exclude:
+    # No point running rubocop with old rest-client
+    - gemfile: .Gemfile-rest-client-1.8.rb
+      env: TASK=rubocop