Update README.md to Linux 4.19

[nosnilmot]

The NDT7 protocol mentions M-Lab uses Linux 4.19. Certain key TCP_INFO struct members are only available in Linux 4.19+ (notably bytes_sent), so for consistency update the README.md to say the same.

btw, the NDT7 protocol also says:

An implementation running on a kernel where a specific TCP_INFO variable mentioned in this specification is missing SHOULD NOT include such variable in the TCPInfo object sent to the client.

but that is not what this implementation does, it sends a full TCPInfo object where missing variables have 0 value.

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This change is 

[coveralls]

Pull Request Test Coverage Report for Build 1642

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 81.463%

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Totals
Change from base Build 1641:	0.0%
Covered Lines:	1815
Relevant Lines:	2228

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💛 - Coveralls