[CI] Add a "prefer-lowest" build on Travis

[mbabker]

It looks like all of the Travis builds are testing with the latest versions of dependencies within whatever constraints are used on a build (i.e. the different Symfony versions or with #182 different versions of the state machine packages). Is there a reason there isn't a build that passes the --prefer-lowest flag into Composer to force a build with the minimum declared dependencies? In part, this helps ensure the minimums are correctly defined.

[ztellman]

Sorry for missing this issue, I'll take a look and see what I can find.

[ztellman]

I believe this is a duplicate of #191. Can you test with beta3 and verify that it's fixed?

[stijnopheide]

Yes, beta3 fixes this. Thanks!