Bump minimum requirements for next vagrant-lxc release (v1.5)

We now depend on vagrant 1.9+ and LXC 2.0+, following bundled version in
debian stretch.

Also, update Gemfile to target latest vagrant/vagrant-spec. It's too
complicated to test against old vagrant versions. Vagrant 1.8.x didn't
specify an upper bound for supported versions but vagrant 1.9 does
(<2.4). This breaks out CI testing matrix which tests against 2.3, 2.4
and 2.5.

So let's make our live easier and just test against the latest vagrant
version and try not to mistakenly drop support for our lowest supported
vagrant version...

Gemfile

@@ -1,18 +1,17 @@
 source 'https://rubygems.org'
 
 group :development do
-  gem 'vagrant', git: 'https://github.com/mitchellh/vagrant.git', tag: 'v1.8.7'
+  gem 'vagrant', git: 'https://github.com/mitchellh/vagrant.git'
   gem 'guard'
   gem 'guard-rspec'
   gem 'rb-inotify'
 end
 
 group :development, :test do
   gem 'rake', '~> 10.4.2'
-  gem 'rspec', '~> 2.99.0'
+  gem 'rspec', '~> 3.5.0'
   gem 'coveralls', '~> 0.7.2', require: (ENV['COVERAGE'] == 'true')
-  # The is the ref *just* before we switch to childprocess 0.6, which conflicts with vagrant 1.8 deps.
-  gem 'vagrant-spec', git: 'https://github.com/mitchellh/vagrant-spec.git', ref: '5006bc73cd8796465ca09307d4774f8ec8375aa0'
+  gem 'vagrant-spec', git: 'https://github.com/mitchellh/vagrant-spec.git'
 end
 
 group :plugins do

README.md

@@ -2,7 +2,7 @@
 
 [![Build Status](https://travis-ci.org/fgrehm/vagrant-lxc.png?branch=master)](https://travis-ci.org/fgrehm/vagrant-lxc) [![Gem Version](https://badge.fury.io/rb/vagrant-lxc.png)](http://badge.fury.io/rb/vagrant-lxc) [![Code Climate](https://codeclimate.com/github/fgrehm/vagrant-lxc.png)](https://codeclimate.com/github/fgrehm/vagrant-lxc) [![Coverage Status](https://coveralls.io/repos/fgrehm/vagrant-lxc/badge.png?branch=master)](https://coveralls.io/r/fgrehm/vagrant-lxc) [![Gitter chat](https://badges.gitter.im/fgrehm/vagrant-lxc.png)](https://gitter.im/fgrehm/vagrant-lxc)
 
-[LXC](http://lxc.sourceforge.net/) provider for [Vagrant](http://www.vagrantup.com/) 1.8+
+[LXC](http://lxc.sourceforge.net/) provider for [Vagrant](http://www.vagrantup.com/) 1.9+
 
 This is a Vagrant plugin that allows it to control and provision Linux Containers
 as an alternative to the built in VirtualBox provider for Linux hosts. Check out
@@ -17,8 +17,8 @@ to see it in action.
 
 ## Requirements
 
-* [Vagrant 1.8+](http://www.vagrantup.com/downloads.html)
-* lxc >=1.0
+* [Vagrant 1.9+](http://www.vagrantup.com/downloads.html)
+* lxc >=2.1
 * `redir` (if you are planning to use port forwarding)
 * `brctl` (if you are planning to use private networks, on Ubuntu this means `apt-get install bridge-utils`)