Fixing composer conflict section to allow twig/twig >=1.41,<2.0

[andkovt]

Constraint defined in composer.json conflict section prevents from using twig versions from 1.41 up to 2.0. Looking at the reason why this constraint was added

• https://github.com/symfony/swiftmailer-bundle/pull/282
• https://github.com/symfony/swiftmailer-bundle/commit/2e5f0d61cf5d3b369c74b18dc0a72a158d95828d

it seems that this is an error and twig/twig >=1.41,<2.0 should be supported by this package since twig 1.41+ supports "filter" https://twig.symfony.com/doc/1.x/filters/filter.html

[fabpot]

Thank you @andkovt.