allowed any Symfony 2.x versions
fixed tests for Symfony 2.6+
simplified the build matrix
minor #1057 allowed any Symfony 2.x versions for Silex 1.2 (fabpot)

This PR was merged into the 1.2 branch.

Discussion
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allowed any Symfony 2.x versions for Silex 1.2

As we know that Symfony 3.0 will be next year, let's allow any Symfony 2.x versions. I've added the 2.6 and 2.7 versions in Travis to check that everything work fine for all supported Symfony 2.x versions.

Commits
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a5dd2b5 simplified the build matrix
ec53f07 fixed tests for Symfony 2.6+
288d891 allowed any Symfony 2.x versions