Remove test files from bundled gem

[t-richards]

Summary

This PR slims down the bundled gem by ~99%.

	Compressed .gem size (bytes)	Unpacked size (bytes)
Before	120,832	581,928
After	6,656	5,418
Difference (absolute)	114,176	576,510
Difference (relative)	-94.49%	-99.07%

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 files from the gem except for lib/ and other individually specified files.

Before (details)

## Compressed .gem size
120832 bytes

## Gem contents
-rw-r--r-- wheel/wheel     154 2019-10-21 23:40 .gitignore
-rw-r--r-- wheel/wheel     228 2019-10-21 23:40 .travis.yml
-rw-r--r-- wheel/wheel     648 2019-10-21 23:40 CHANGELOG.md
-rw-r--r-- wheel/wheel      88 2019-10-21 23:40 Gemfile
-rw-r--r-- wheel/wheel    1287 2019-10-21 23:40 LICENSE
-rw-r--r-- wheel/wheel     772 2019-10-21 23:40 README.md
-rwxr-xr-x wheel/wheel     482 2019-10-21 23:40 Rakefile
-rwxr-xr-x wheel/wheel     691 2019-10-21 23:40 ext/mkrf_conf.rb
-rw-r--r-- wheel/wheel     698 2019-10-21 23:40 lib/unf.rb
-rw-r--r-- wheel/wheel     800 2019-10-21 23:40 lib/unf/normalizer.rb
-rw-r--r-- wheel/wheel     521 2019-10-21 23:40 lib/unf/normalizer_cruby.rb
-rw-r--r-- wheel/wheel     608 2019-10-21 23:40 lib/unf/normalizer_jruby.rb
-rw-r--r-- wheel/wheel      41 2019-10-21 23:40 lib/unf/version.rb
-rw-r--r-- wheel/wheel     406 2019-10-21 23:40 test/helper.rb
-rw-r--r-- wheel/wheel  571528 2019-10-21 23:40 test/normalization-test.txt
-rw-r--r-- wheel/wheel    1671 2019-10-21 23:40 test/test_unf.rb
-rw-r--r-- wheel/wheel    1305 2019-10-21 23:40 unf.gemspec

## Unpacked size
581928 bytes

After (details)

## Compressed .gem size
6656 bytes

## Gem contents
-rw-r--r-- wheel/wheel    1287 2019-10-21 23:40 LICENSE
-rw-r--r-- wheel/wheel     772 2019-10-21 23:40 README.md
-rwxr-xr-x wheel/wheel     691 2019-10-21 23:40 ext/mkrf_conf.rb
-rw-r--r-- wheel/wheel     698 2019-10-21 23:40 lib/unf.rb
-rw-r--r-- wheel/wheel     800 2019-10-21 23:40 lib/unf/normalizer.rb
-rw-r--r-- wheel/wheel     521 2019-10-21 23:40 lib/unf/normalizer_cruby.rb
-rw-r--r-- wheel/wheel     608 2019-10-21 23:40 lib/unf/normalizer_jruby.rb
-rw-r--r-- wheel/wheel      41 2019-10-21 23:40 lib/unf/version.rb

## Unpacked size
5418 bytes