Deprecate PHP 7.1, 7.2, support 7.4

[bagofarms]

Now that we have support for PHP 7.3, and 7.1 and 7.2 are no longer actively updated (7.2 gets security fixes only through November), we need to:

• Remove PHP 7.1 and 7.2 support from documentation
• Remove Travis tests for anything older than 7.3
• Add Travis tests for 7.3 and 7.4
• Make any code updates required
• Update Composer and packages as required

[lukeislucas]

Is there any timeline on this item? We are trying to install this on a PHP 7.4 server and ran into issues.

[bagofarms]

I expect to start working on this over the next couple of weeks, but I don't know what kinds of issues we'll run into. Could you provide some more information on the issues you encountered?

[lukeislucas]

When running the install via php composer.phar install, the following error stops the installation:

Your requirements could not be resolved to an installable set of packages.
Problem 1
- This package requires php ~7.2.0 || ~7.3.0 but your PHP version (7.4.6) does not satisfy that requirement.

We are in the process of migrating a working version from PHP 7.2 over to our PHP 7.4 server, but we have not had a chance to test success in that scenario yet.

[bagofarms]

Related to #546

[bagofarms]

Some of this was done in #559