Don't require that the env be specified

[GregMefford]

When working on the spandex_datadog sender, I realized that we don't currently have a way to not send the env with the spans. With Datadog, if you don't specify an env in the application like this, it will use the env configured in the trace collector agent. This PR makes it possible to do so, and I have another PR ready on the spandex_dagtadog repo that supports this from that end.

[coveralls]

Pull Request Test Coverage Report for Build 328

• 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 82.278%

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Totals
Change from base Build 319:	0.0%
Covered Lines:	195
Relevant Lines:	237

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💛 - Coveralls

[zachdaniel]

Merge at your liesure.

[GregMefford]

Coveralls won't let me. 😭 I think it was confused because the Travis build failed the first time and I re-ran it but coveralls didn't update its status.

[GregMefford]

Ok, amended the commit and got it to give me a green check, but I still can't merge due to user permissions.