Simplified the Travis build matrix

Testing the older releases and the dev versions of vendors separately
and against all PHP versions is wasting the Travis resources and making
builds longer given that the number of concurrent jobs is limited.
Testing these libraries on each PHP version is not needed (they already
do it) and they can be combined in the same build.
This only going down from 20 jobs per build to 6.
Changed dependencies to stable by default for the testsuite

This also update PHPUnit to the current version rather than the old 3.7.
Fixed wrong assertion
Fixed the testing of dev versions

Instead of forcing only a few dev versions, which then require
whitelisting their own dependencies recursively in some case, the
minimum stability is altered so that semver constraint will all match
dev versions.
Merge pull request #150 from stof/simplify_travis

Simplified the Travis build matrix