DDC-3837: [GH-1468] Travis: Enable cache for composer

[doctrinebot]

Jira issue originally created by user @doctrinebot:

This issue is created automatically through a Github pull request on behalf of Majkl578:

Url: #1468

Message:

Travis is now able to persist caches and reuse them in future builds. This could avoid re-downloading of stable packages (5 atm).
As @stof suggested in #1466, after removing --prefer-source, stable versions will be downloaded as an archive while dev versions will be still cloned by git directly.
More info in Caching Dependencies and Directories.

[doctrinebot]

Comment created by @guilhermeblanco:

Duplicate of [DDC-3989](http://www.doctrine-project.org/jira/browse/[DDC-3989]%28http://www.doctrine-project.org/jira/browse/DDC-3989%29)

[doctrinebot]

Issue was closed with resolution "Duplicate"

[doctrinebot]

Comment created by @doctrinebot:

A related Github Pull-Request [GH-1468] was closed:
#1468

[doctrinebot]

Comment created by @doctrinebot:

A related Github Pull-Request [GH-1468] was assigned:
#1468