Reference stable GoogleChromeLabs/third-party-capital release (or at least main branch)

[felixarntz]

Currently the library references the php-support branch of GoogleChromeLabs/third-party-capital.

Once GoogleChromeLabs/third-party-capital#40 has been merged, this should be changed - at a minimum to reference the main branch.

Additionally, once GoogleChromeLabs/third-party-capital has been published to Packagist, the repositories entry should be dropped from this composer.json file, to consume the project from Packagist rather than directly from GitHub.

Even better would be to publish an actual release of the library, e.g. a 0.1.0.

cc @adamsilverstein @housseindjirdeh

[felixarntz]

This is now fixed. wp-third-parties should be ready for an initial stable release. cc @adamsilverstein