Change constraint generated by migration.

Previous the constraint select a specific version+build. But builds get
lost in the ER2 when updating the release. Now all updates including
minor updates are allowed by convention.
Make database update more colorful.
Add stability to lower level version in constraints.
Update required contao version in readme.
Fix install sources from vcs.
Make console output more readable.
Update composer.json.
fix of 2 bugs when checking VCS requirements

1. snvAvailable variable was misspelled therefor not found
2. proc_close() sometimes retunres 1 which is OK but  the implementation
was converting it to false on VCS requirement check. I changed it to
check if the result is different than -1 which is the only case of error
according to the documentation:
http://php.net/manual/en/function.proc-close.php
Merge pull request #98 from sapeish/check-vcs

fix of 2 bugs when checking VCS requirements
Always load swift library in Contao 2.
Fix could not load Swift_DependencyContainer issue.
Introduce namespaces with custom class loader.
Extract download functionality.
Fix missing global namespace prefix.
Extract convenience method Runtime::updateComposer().
Extract convenience method Runtime::increaseMemoryLimit().
Extract convenience method Runtime::readComposerDevWarningTime().
Extract convenience method Runtime::clearComposerCache().
Extract convenience method Runtime::isSafeModeHackEnabled().
Extract convenience method Runtime::isPhpVersionSupported().