Update gemspec to follow using nokogiri

Correct how to run test

Follow d75f3ee5

Its absolutely a hack, but that's what I have the energy for right
now.

Closes #108, #106

[ex/mimemagic/Rakefile] Use older Ruby HEREDOC

The "squiggly" heredoc was introduced in later versions of Ruby, so
using a more compatible version of them with gsubing (to have the same
effect) to allow this to build on more versions of Ruby.

Fix mime type database package name in docs

Installing this gem on a system which does NOT contain the external
mime-type database fails indicating that the user should install the
shared-mime-types package.

```
Ensure you have either installed the shared-mime-types package for your distribution, or
obtain a version of freedesktop.org.xml and set FREEDESKTOP_MIME_TYPES_PATH to the location
of that file.
/usr/local/bundle/ruby/2.7.0/gems/mimemagic-0.3.7/ext/mimemagic/Rakefile:14:in `locate_mime_database'
/usr/local/bundle/ruby/2.7.0/gems/mimemagic-0.3.7/ext/mimemagic/Rakefile:25:in `block in <top (required)>'
/usr/local/bundle/ruby/2.7.0/gems/rake-13.0.3/exe/rake:27:in `<main>'
Tasks: TOP => default
(See full trace by running task with --trace)

rake failed, exit code 1

Gem files will remain installed in /usr/local/bundle/ruby/2.7.0/gems/mimemagic-0.3.7 for inspection.
Results logged to /usr/local/bundle/ruby/2.7.0/extensions/x86_64-linux/2.7.0/mimemagic-0.3.7/gem_make.out

An error occurred while installing mimemagic (0.3.7), and Bundler cannot continue.
Make sure that `gem install mimemagic -v '0.3.7' --source 'https://rubygems.org/'` succeeds before bundling.
```

Other places in the documentation this package is referred to as
`shared-mime-info`. This changes the error message to be consistent with
the documentation as to reduce confusion for the consumer.

This will ensure that end-users who are requiring that file don't
have runtime errors.

Resolve issues around GPL compliance by making use of an externally provided database. Revert to MIT license.

Because we're required to build a C extension in order to do so
(don't ask, its a long story), use that C extension to make the
path provided at build time available at run time.

Currently looks at the value of `FREEDESKTOP_MIME_TYPES_PATH` and
in `/usr/share/mime/packages/freedesktop.org.xml`, which is the
path you would expect to find that file at on a typical Linux
system.

This is the first step in removing the dependency on distributing
freedesktop.og.xml as part of the gem.

This allows it to be called at runtime.