Stop calling doi.org

We've experienced some performance issues when calling doi.org API.

Fix guzzle Exception error

0.6.2

Apply fixes from StyleCI (#26)

update Carbon

update Carbon

guzzle-factory update

guzzle-factory update

PHP 8 and higher

Allow for PHP 8 and higher

Fetch complementary resources bug

There is a bug in the complementary resources fetch process.

Resolve Carbon conflict

0.4.2

Didn't like Carbon update on an old version of Laravel

Preprint data

Add a link key on the generated JSON to reflect the publication's link to a preprint. Two values are possible:

• is_preprint_of
• has_preprint

symfony/dom-crawler upgrade

Last upgrade needed: symfony/dom-crawler

Package updates

Updated Carbon to a newer version and fixed a couple of deprecation warnings as well as PHP 7.3 incompatibilities.

Also now testing for PHP 7.3 and PHP 7.4 in TravisCI