Added a complete Symfony version checker to decide if we can install it
feature #29 Added a complete Symfony version checker to decide if we can install it (javiereguiluz)

This PR was squashed before being merged into the 1.0-dev branch (closes #29).

Discussion
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Added a complete Symfony version checker to decide if we can install it

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #25
| License       | MIT
| Doc PR        | -

Commits
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6b323fc Added a complete Symfony version checker to decide if we can install it