Fix flaky CI tests

[chrisradek]

The TestLambdaSegmentEmit test in lambda_test.go appears to be flaky:
https://travis-ci.org/aws/aws-xray-sdk-go/jobs/510026094
This test has failed a couple times in different builds since we reactivated travis.

We've also seen an invalid memory address or nil pointer dereference error in the TestMySQL test once:
https://travis-ci.org/aws/aws-xray-sdk-go/jobs/510026091

[luluzhao]

Update:

SQL unit test is fixed by this PR: #121

still need to get Lambda unit test fixed.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.