Fix Ruby and Gem versions for the build environment (取敢えず)

.gitignore

@@ -1 +1,2 @@
+/Gemfile.lock
 /tmp/

.ruby-version

@@ -0,0 +1 @@
+2.3.1

Gemfile

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+source 'https://rubygems.org'
+
+gem 'net-ssh', '>= 3.2.0'

README.md

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+Demonstrate and Play with Ruby's Net::SSH
+=========================================
+
+#### Environment
+
+I run Ruby from [rbenv](https://github.com/rbenv/rbenv); I have a
+`.ruby-version` file here indicating version 2.3.1 because that
+happens to be an environment where this will likely be used. In this
+project I also use [Bundler](https://bundler.io/) to get the correct
+versions of Gems.
+
+After setting up `rbenv` and installing 2.3.1, you can set up the rest
+of the environment with the usual:
+
+    gem update --system
+    gem install bundler
+    bundle install
+
+#### Net::SSH Version
+
+I currently use 3.2.0, the latest stable version. Depending on how
+things progress both on my exploration of the 3.2 API and the release
+process of 4.0.0, I may change to using 4.x sooner or later.