changed the license from LGPL to MIT
merged branch swiftmailer/mit (PR #319)

This PR was merged into the master branch.

Discussion
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changed the license from LGPL to MIT

Commits
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b302c1f changed the license from LGPL to MIT
fixed version in composer.json
prepared the 5.0.0 release
bumped version to 5.1.0-DEV
updated dev version to 5.1
fixed LICENSE file
fixed compatbility with PHP 5.2
udpated CHANGELOG