allow to make puppet composer module optional

[c33s]

willdurand/composer a hard requirement but it only provides the composer executeable. this file can be provided by other things like puppet/php or in my case by my profile::php.
to allow the user of this module to use it without requiring willdurand/composer it should be optional. which this PR is for.

[tohuwabohu]

Can you please provide a bit more context for this change? I'm not objecting it, just curious to understand the reason behind.

[c33s]

updated description. the ubuntu tests are failing because of a download problem of the roundcube archive, are they? is it possible to retrigger the tests?

[tohuwabohu]

@c33s You can trigger a new build on the travis CI page via the "more options", top right corner.

[c33s]

not possibe because i have no write access to this repo. https://stackoverflow.com/a/17624403

will try the close and reopen tactic :)

[tohuwabohu]

not possibe because i have no write access to this repo. https://stackoverflow.com/a/17624403

will try the close and reopen tactic :)

Ah dang, didn't know about that. Glad the workaround worked for you, will pay more attention to this next time.

[tohuwabohu]

Released as 3.4.0 to the forge, thanks heaps for the contribution.