Remove `--ignore-platform-reqs` from Composer commands in Travis builds

[dshoreman]

Once updates are available to add php 8 support (see below), $COMPOSER_OPTS can be removed from the install command along with the one remaining env: COMPOSER_OPTS... line in .travis.yml:

ShellCommandBuilder/.travis.yml

Lines 11 to 20 in 2497b8f

	- php: nightly
	dist: bionic
	env: COMPOSER_OPTS="--ignore-platform-reqs"
	cache:
	directories:
	- $HOME/.composer/cache
	install:
	- travis_retry composer install $COMPOSER_OPTS

Before this can happen, some deps need to be updated. That's partially beyond our control, so it was left out of #17.

Required changes upstream

As far as I can tell, the only updates we're waiting for are in phpunit-watcher, grumphp-config and (by proxy) grumphp.

spatie/phpunit-watcher

PHP 8 support is merged, but not yet released. #17 sets the requirement to "^1.24 || dev-master#071fbbf" as a workaround.

Assuming the next version is 1.24, we can drop || dev-master#... once it's released.

pluswerk/grumphp-config

This was updated for composer 2 support in 4.0.1, but upstream currently lacks PHP 8 support.

• pluswerk/grumphp-config should require grumphp ^0.19 || ^1.X to maintain 7.2 support
  (replace X with whichever version adds php 8 support - 1.1 doesn't have it yet)
• phpro/grumphp needs to allow >=7.3 or ^7.3 || ^8.0, but currently locks to ^7.3