postgres: Make ensureVersionTable atomic

[tsenart]

Fixes #55.

[tsenart]

As you can see in this build log, this test reproduces what we see in #55 and https://github.com/sourcegraph/sourcegraph/issues/1491.

I'm proposing a fix here which I'm not 100% sure about. Please advise.

[coveralls]

Pull Request Test Coverage Report for Build 267

• 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 59.188%

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Totals
Change from base Build 264:	0.0%
Covered Lines:	773
Relevant Lines:	1306

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 267

• 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 59.188%

---

Totals
Change from base Build 264:	0.0%
Covered Lines:	773
Relevant Lines:	1306

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💛 - Coveralls

[tsenart]

Closing to re-open from a branch in my fork. I need the master branch to rename the import paths so that we can use the fork in the mean time.