Add unit test for --disallow-code-generation-from-strings

[davidje13]

Alternative testing approach based on discussion in #41

This introduces a failing test for Node 9+. This is separate from the fix so that it can be combined with any of the existing PRs.

I have tested the test with the code from #42 and it passes, as expected.

[davidje13]

@dougwilson I took the approach you suggested. This certainly simplifies things and means it can still support Node 0.8 (no need for the cross-env package).

As noted in the description, this is an isolated test change so that it can be combined with any of the 3 PRs which offer fixes (#26 / #33 / #42), or another approach.

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btw, since Node 5.12 is consistently failing in Travis but not Appveyor, I'd suggest it's probably a bad cache. Are you able to clear the Travis cache for 5.12? The only other option I can think of is that browserify has somehow broken on Node 5.12 but only on Linux.