Added a complete Symfony version checker to decide if we can install it
minor #57 Fixed a visual glitch in the progress bar (javiereguiluz)

This PR was merged into the 1.0-dev branch.

Discussion
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Fixed a visual glitch in the progress bar

I don't know if others suffered this problem, but on my computer (Mac OS + iTerm 2) there was a visual glitch in the progress bar that wrongly showed two percent signs when the download size goes from KB to MB:

**Before**

![symfony_installer_before](https://cloud.githubusercontent.com/assets/73419/5180762/dddfc204-748f-11e4-9444-277de8839b03.gif)

**After**

![symfony_installer_after](https://cloud.githubusercontent.com/assets/73419/5180763/e02bc288-748f-11e4-8061-7e03138ab4b2.gif)

Commits
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9eccc30 Added a complete Symfony version checker to decide if we can install it