Rename vendor to third_party

[SaiedKazemi]

This commit renames vendor to third_party because the usage of the vendor directory in this repo is not consistent with Go's conventions for the vendor directory (https://golang.org/src/README.vendor). Specifically, the vendor directory contains two third-party packages written in C, scamper and libparistraceroute, where the first one is included as a source tarball and the second one as a Git submodule.

The changes were successfully tested by running:

docker build -t traceroute-caller:latest .

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This change is 

[coveralls]

Pull Request Test Coverage Report for Build 87

• 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 97.626%

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Totals
Change from base Build 70:	0.0%
Covered Lines:	1069
Relevant Lines:	1095

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💛 - Coveralls

[stephen-soltesz]

Please double check the docker build error.