Update dependency symfony/flex to v2

This MR contains the following updates:

Package	Type	Update	Change
symfony/flex	require	major	^1.11 -> ^2.0

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Release Notes

symfony/flex

v2.1.4

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v2.1.3

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v2.1.2

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v2.1.1

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v2.1.0

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v2.0.1

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v2.0.0

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Configuration

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