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)