use semver caret operator and stable symfony 3

[Tobion]

@fabpot would be nice to merge this before tagging 3. all those libraries use semantic versioning. so lets use caret operator to spreads best practices.

[fabpot]

Thank you @Tobion.

[ghost]

@Tobion Could you explain the different between tildes (~) and carets (^)? I don't really understand the difference in the official composer documentation. https://getcomposer.org/doc/articles/versions.md#caret

An example of the difference would be nice. As I understand both ensure backward compatibility because they do not allow changes of the major version.

[stof]

@JHGitty if you use only a major or minor version after the operator, there is no difference. But if you specify a patch version, ~1.3.2 will go only up to the next minor version (<1.4), i.e. it allows to increment only the last segment (excluding major versions), whereas ^1.3.2 will still allow to move up to the next major version (<2.0)