adds support for PTY mode
adds convenience method mustRun
attempts to fix tests on Travis
adds cache for isPtySupported()
feature #8655 Adds PTY mode & convenience method mustRun() (schmittjoh)

This PR was merged into the 2.5-dev branch.

Discussion
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Adds PTY mode & convenience method mustRun()

This makes two additions. I've split them into separate commits so that you can pull them separately if required.

1. Adds PTY mode.
In contrast to the existing TTY mode, the proc_open call becomes the master not the process currently executing.

2. Adds a ``mustRun`` method
This is merely for convenience:

```php
# Before
$proc = new Process($cmd);
if (0 !== $proc->run()) {
    throw new ProcessFailedException($proc);
}

$proc = new Process($cmd);
if (0 !== $proc->run()) {
    throw new ProcessFailedException($proc);
}

# After
(new Process($cmd))->mustRun();
(new Process($cmd))->mustRun();

Commits
-------

dbd264a adds cache for isPtySupported()
6c11207 attempts to fix tests on Travis
2ff1870 adds convenience method mustRun
53441aa adds support for PTY mode
removed unneded test groups