Enable test with Python3.7

[luzfcb]

Issue:
#156

In November of last year, AWS Lambda included official support for Python3.7 on Lambda, however, this project is not being tested using Python3.7.

Description of changes:

• Change the Travis-ci configuration to use Ubuntu Xenial (16.04) instead of Ubuntu Trusty (14.04) as base OS to allow run the tests with Python3.7. Python 3.7 is not available on Ubuntu 14.04, and 14.04 already reached the End of Life status.
• Update the 'tox' configurations and Travis-ci configuration to enable test with Python3.7

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[luzfcb]

The tests fail but are unrelated to my changes

[awssandra]

We've run into some flakey test issues before. We'll take a look to see what's going on.

[chanchiem]

Investigating the flaky test to see if we can get that resolved. It seemed to have failed in pr #158 as well

[chanchiem]

Tested this PR locally and all tests seem to pass except the flaky one; it seems to be failing for all versions. Will need to investigate flaky tests as a separate issue. Although it's strange that it seemed to have failed through all the versions. Perhaps the failed test is an indication of an actual bug.

Another thing is that for some reason Travis CI isn't able to download Python 3.7 and run the test for that version either. There does seem to be a workaround here but we will have to investigate further to make sure everything works correctly. This PR in itself looks fine and will be pushed as soon as we can get these other issues fixed.

We really appreciate your PR and hope to get these issues resolved as soon as possible.

Edit:
After rerunning the travis CI on the latest stable release commit, the same test seems to be failing now when it used to be passing.
Test just ran: https://travis-ci.org/aws/aws-xray-sdk-python/builds/502178680
Last test ran: https://travis-ci.org/aws/aws-xray-sdk-python/builds/502170147

These are both for the same branch but the first one was ran a few minutes ago while the latter was ran several months ago. Perhaps there was a modification in one of our dependencies

[luzfcb]

@chanchiem I changed the configuration to make Travis-ci use Ubuntu 16.04 instead of 14.04 to run the tests.

[chanchiem]

Everything looks good! Thanks for your contribution!

[chanchiem]

Support for 3.7 Everything looks good!

[chanchiem]

Had tested on local Python test as well as on Lambda through Serverless and manual X-Ray invocation.