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.

Reduce gem package size

Ensure that frozen string literals used throughout the code

Remove rubyforge_project

This is deprecated by rubygems:
```
NOTE: Gem::Specification#rubyforge_project= is deprecated with no replacement. It will be removed on or after 2019-12-01.

Gem::Specification#rubyforge_project= called from /home/travis/build/sul-dlss/pre-assembly/vendor/bundle/ruby/2.5.0/specifications/mimemagic-0.3.3.gemspec:16.
```

Frozen string literal

Handle Office Open XML types for filed generated outside MSOffice

CI: rbx-3 in allow_failures, comment on Bundler

Travis: Use Bundler < 2

README: Use GitHub Markdown code fences

README: Use SVG badges