[Process] Renamed flushOutput() and flushErrorOutput() to clearOutput() and clearErrorOutput()

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

I renamed `flushOutput()` and `flushErrorOutput()` to `clearOutput()` and `clearErrorOutput()` since I find the current naming unintuitive. When reading [the blog post about this feature](http://symfony.com/blog/new-in-symfony-2-4-flushing-stdout-and-stderr-on-a-process) I initially thought that the output would be flushed to the console (as in `ob_flush()`).

Using "clear" on the other hand conforms well with [our conventions](http://symfony.com/doc/current/contributing/code/conventions.html).