textproc/ruby-erubi: add version 1.7.1 package

Erubi is a ERB template engine for ruby. It is a simplified fork of Erubis,
using the same basic algorithm, with the following differences:

* Handles postfix conditionals when using escaping
  (e.g. <tt><%= foo if bar %></tt>)
* Supports frozen_string_literal: true in templates via :freeze option
* Works with ruby's --enable-frozen-string-literal option
* Automatically freezes strings for template text when ruby optimizes it (on
  ruby 2.1+)
* Escapes ' (apostrophe) when escaping for better XSS protection
* Has 6x faster escaping on ruby 2.3+ by using cgi/escape
* Has 86% smaller memory footprint
* Does no monkey patching (Erubis adds a method to Kernel)
* Uses an immutable design (all options passed to the constructor, which
  returns a frozen object)
* Has simpler internals (1 file, <150 lines of code)
* Has an open development model (Erubis doesn't have a public source control
  repository or bug tracker)
* Is not dead (Erubis hasn't been updated since 2011)

It is not designed with Erubis API compatibility in mind, though most Erubis
ERB syntax works, with the following exceptions:

* No support for <tt><%===</tt> for debug output

(taca)