Add test for pool's TestOnBorrow locking.

When I read `Pool.Get()` earlier today, I thought there was a locking bug because it had lock/unlock inside a for loop. I wrote this test to verify that the bug exists before fixing, but I turned out to be mistaken. Your Pool locking is solid. But I thought you might benefit from this test I wrote, so you don't have any regressions in the future.
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Merge pull request #93 from jart/patch-1

Add test for pool's TestOnBorrow locking.
Merge pull request #94 from joshk/patch-1

Use the new build env on Travis