Update ruby-sinatra to 2.0.8.1.


## 2.0.8.1 / 2020-01-02

* Allow multiple hashes to be passed in `merge` and `merge!` for `Sinatra::IndifferentHash` [#1572](sinatra/sinatra#1572) by Shota Iguchi

## 2.0.8 / 2020-01-01

* Lookup Tilt class for template engine without loading files [#1558](sinatra/sinatra#1558). Fixes [#1172](sinatra/sinatra#1172) by Jordan Owens

* Add request info in NotFound exception [#1566](sinatra/sinatra#1566) by Stefan Sundin

* Add `.yaml` support in `Sinatra::Contrib::ConfigFile` [#1564](sinatra/sinatra#1564). Fixes [#1563](sinatra/sinatra#1563) by Emerson Manabu Araki

* Remove only routing parameters from @params hash [#1569](sinatra/sinatra#1569). Fixes [#1567](sinatra/sinatra#1567) by Jordan Owens, Horacio

* Support `capture` and `content_for` with Hamlit [#1580](sinatra/sinatra#1580) by Takashi Kokubun

* Eliminate warnings of keyword parameter for Ruby 2.7.0 [#1581](sinatra/sinatra#1581) by Osamtimizer

## 2.0.7 / 2019-08-22

* Fix a regression [#1560](sinatra/sinatra#1560) by Kunpei Sakai

## 2.0.6 / 2019-08-21

* Fix an issue setting environment from command line option [#1547](sinatra/sinatra#1547), [#1554](sinatra/sinatra#1554) by Jordan Owens, Kunpei Sakai

* Support pandoc as a new markdown renderer [#1533](sinatra/sinatra#1533) by Vasiliy

* Remove outdated code for tilt 1.x [#1532](sinatra/sinatra#1532) by Vasiliy

* Remove an extra logic for `force_encoding` [#1527](sinatra/sinatra#1527) by Jordan Owens

* Avoid multiple errors even if `params` contains special values [#1526](sinatra/sinatra#1527) by Kunpei Sakai

* Support `bundler/inline` with `require 'sinatra'` integration [#1520](sinatra/sinatra#1520) by Kunpei Sakai

* Avoid `TypeError` when params contain a key without a value on Ruby < 2.4 [#1516](sinatra/sinatra#1516) by Samuel Giddins

* Improve development support and documentation and source code by  Olle Jonsson, Basavanagowda Kanur, Yuki MINAMIYA