Remove test files from bundled gem

[t-richards]

Summary

This PR slims down the bundled gem by 80%.

	Gem Size (bytes)
Before	92,160
After	18,432
Difference (absolute)	73,728
Difference (relative)	-80.00%

I believe these savings are worth having because this gem is downloaded millions of times.

Changes

• Removed the test_files declaration from the gemspec (because it is "no longer useful or recommended to set")
• Omitted all test files from the gem.

Before (details)

## Gem size
-rw-r--r-- 1 tom users 92160 Oct 15 22:57 pkg/crass-1.0.4.gem

## Gem contents
.gitignore
.travis.yml
.yardopts
Gemfile
HISTORY.md
LICENSE
README.md
Rakefile
crass.gemspec
lib/crass.rb
lib/crass/parser.rb
lib/crass/scanner.rb
lib/crass/token-scanner.rb
lib/crass/tokenizer.rb
lib/crass/version.rb
test/css-parsing-tests/An+B.json
test/css-parsing-tests/LICENSE
test/css-parsing-tests/README.rst
test/css-parsing-tests/color3.json
test/css-parsing-tests/color3_hsl.json
test/css-parsing-tests/color3_keywords.json
test/css-parsing-tests/component_value_list.json
test/css-parsing-tests/declaration_list.json
test/css-parsing-tests/make_color3_hsl.py
test/css-parsing-tests/make_color3_keywords.py
test/css-parsing-tests/one_component_value.json
test/css-parsing-tests/one_declaration.json
test/css-parsing-tests/one_rule.json
test/css-parsing-tests/rule_list.json
test/css-parsing-tests/stylesheet.json
test/css-parsing-tests/stylesheet_bytes.json
test/shared/parse_rules.rb
test/support/common.rb
test/support/serialization/animate.css
test/support/serialization/bootstrap-theme.css
test/support/serialization/bootstrap.css
test/support/serialization/html5-boilerplate.css
test/support/serialization/misc.css
test/support/serialization/pure.css
test/test_crass.rb
test/test_css_parsing_tests.rb
test/test_parse_properties.rb
test/test_parse_rules.rb
test/test_parse_stylesheet.rb
test/test_serialization.rb

After (details)

## Gem size
-rw-r--r-- 1 tom users 18432 Oct 15 22:57 pkg/crass-1.0.4.gem

## Gem contents
.gitignore
.travis.yml
.yardopts
Gemfile
HISTORY.md
LICENSE
README.md
Rakefile
crass.gemspec
lib/crass.rb
lib/crass/parser.rb
lib/crass/scanner.rb
lib/crass/token-scanner.rb
lib/crass/tokenizer.rb
lib/crass/version.rb

[rgrove]

Thanks!