[modify] modified Rack gem version

[nikukyugamer]

Hi! table-setter is beautiful product, thank you :)

btw, we must specify Rack gem version < 2.0.0. Because of changing name from showexception to show_exception in Rack >= 2.0.0.

for example

not specify Rack version

$ bundle install
Fetching rack 2.0.4
Installing rack 2.0.4
$ bundle exec table-setter --help
Traceback (most recent call last):
	5: from /home/takiya/table_setter/vendor/bundle/ruby/2.5.0/bin/table-setter:25:in `<main>'
	4: from /home/takiya/table_setter/vendor/bundle/ruby/2.5.0/bin/table-setter:25:in `load'
	3: from /home/takiya/table_setter/vendor/bundle/ruby/2.5.0/gems/table_setter-0.2.12/bin/table-setter:5:in `<top (required)>'
	2: from /home/takiya/table_setter/vendor/bundle/ruby/2.5.0/gems/table_setter-0.2.12/bin/table-setter:5:in `require'
	1: from /home/takiya/table_setter/vendor/bundle/ruby/2.5.0/gems/table_setter-0.2.12/lib/table_setter/command.rb:3:in `<top (required)>'
/home/takiya/table_setter/vendor/bundle/ruby/2.5.0/gems/table_setter-0.2.12/lib/table_setter/command.rb:3:in `require': cannot load such file -- rack/showexceptions (LoadError)

specify Rack version < 2.0.0

$ bundle install
Fetching rack 1.6.8
Installing rack 1.6.8
$ bundle exec table-setter --help
table-setter is a Sinatra application for rendering and processing CSVs from google docs into HTML.

Usage:
  table-setter COMMAND path/to/table-setter/assets OPTIONS

commands:
  start    run the development server, for deployment use config.ru
  install  copy the table-setter assets into the the directory
  build    statically build tables in the ./out/ directory

options:
    -p, --prefix PREFIX              url prefix for the export command

great.