updating attr to >=17.4.0 and changing all uses of attr.ib from deprecated "convert" to "converter"#40

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mattsb42-aws merged 2 commits intoaws:masterfrom

mattsb42-aws:dev-39

Jan 12, 2018

Commits on Jan 12, 2018

updating pytest to >=3.3.1 since it addressed the 3.3.0 problem
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updating attr to >=17.4.0 and changing all uses of attr.ib from deprecated "convert" to "converter" https://github.com/awslabs/aws-encryption-sdk-python/issues/39
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