Bring travis up to speed with latest vagrant

- Fix gemfile sourced twice issue: https://github.com/bundler/bundler/issues/5263
- Use latest vagrant-spec
- Fix travis gem issue "no implicit conversion of nil into String"

.travis.yml

@@ -3,26 +3,21 @@ language: ruby
 before_install:
   - sudo apt-get update -qq
   - sudo apt-get install -y libvirt-dev
-  - gem update --system 2.4.8
-  - gem install bundler --version $BUNDLER_VERSION
-install: bundle _${BUNDLER_VERSION}_ install
-script: bundle _${BUNDLER_VERSION}_ exec rspec --color --format documentation
+  - gem update --system
+  - gem install bundler
+install: bundle install
+script: bundle exec rspec --color --format documentation
 notifications:
   email: false
 rvm:
-  - 2.2.3
+  - 2.2.5
+  - 2.3.3
 env:
   global:
     - NOKOGIRI_USE_SYSTEM_LIBRARIES=true
   matrix:
-    - VAGRANT_VERSION=v1.8.4 BUNDLER_VERSION=1.12.5
+    - VAGRANT_VERSION=v1.9.7 
 matrix:
-  include:
-    - env: VAGRANT_VERSION=master BUNDLER_VERSION=1.12.5
-      rvm: 2.2.3
-    - env: VAGRANT_VERSION=v1.7.4 BUNDLER_VERSION=1.7.9
-      rvm: 2.0.0
   allow_failures:
-    - env: VAGRANT_VERSION=master BUNDLER_VERSION=1.12.5
-      rvm: 2.2.3
-
+    - env: VAGRANT_VERSION=master 
+      rvm: 2.3.3

Gemfile

@@ -14,14 +14,13 @@ group :development do
     gem 'vagrant', :git => 'https://github.com/mitchellh/vagrant.git'
   end
 
-  gem 'vagrant-spec', :github => 'mitchellh/vagrant-spec',
-    tag: ENV['VAGRANT_SPEC_VERSION'] || "9bba7e1228379c0a249a06ce76ba8ea7d276afbe"
+  gem 'vagrant-spec', :github => 'mitchellh/vagrant-spec'
 
   gem 'pry'
 end
 
 group :plugins do
-  gem 'vagrant-libvirt', :path => '.'
+  gemspec
 end
 
 gem 'coveralls', require: false