Use a cell's specified font to determine text cell content width. prawnpdf#42

* Make it clear that Prawn license is a bit different to Ruby license
* Use recommented names for GPL

Fixed Rubygems licenses

Update to RSpec 3

Updated Travis CI config

Drop Mocha

Update FSF address

This PR gets CI running with GitHub Actions.  It uses a slightly modified version of the prawnpdf/prawn config file.

It also:

1. Includes a fixture file originally imported with the prawn gem.  Paths in specs were updated
2. Addresses a couple of keyword argument ambiguities
3. Updates the CI badge in the README
4. Conditionally adds the matrix gem to the gemspec when Ruby version is 3.1.0 or higher, as matrix is no longer bundled with Ruby

Thanks to @leoarnold for his original PR and driving the CI change.

…or when value is passed in options

This defines an error constant in prawn-table if it is not already defined.  This allows us to safely remove it from prawn, and for the gem to work with versions of prawn that define it (or not)

Previously these images were being pulled from the prawn gem, but they are no longer included in the gem build.  Updated the images to be included in this repo, and updated the paths.

With this PR the manual builds with no errors.

With the merging of various sources for CI contributions, the badge name doesn't match the workflow name so the badge doesn't show.  This corrects the name of the badge

Sets minimum Ruby version to 2.6.  Adds a Rubocop todo that can be burned down. Adds a code style job to CI

  by the table cell_style
  prawnpdf#56

In the documentation there was a reference to an old example.
In this commit we remove it, because it's no longer valid.

Merge remote-tracking branch 'upstream/master'