[2.4] travis: run coverage just once

[TomasVotruba]

Another speedup, just for 2.4 branch. Ref: #1251

2.3 and bellow don't use coverage, so that should be all.

[doctrinebot]

Hello,

thank you for creating this pull request. However did not open it on the "master"
branch. Our Git workflow requires all pull requests to go through "master" branch
and the release masters then merge them back into stable branches, if they are
bug fixes.

Please open the pull request again for the "master" branch and close
this one.

Nevertheless I have opened a Jira ticket for this Pull Request to track this
issue:

http://www.doctrine-project.org/jira/browse/DDC-3530

We use Jira to track the state of pull requests and the versions they got
included in.

[Ocramius]

@TomasVotruba thanks!

[Ocramius]

@TomasVotruba if I may suggest, it may be interesting to also disable XDebug in some runs (should double the speed as well, especially for composer installation).