Add linux gcc tests to travis config.

[seantalts]

Unclear why we removed it; see this thread for discussion.
http://discourse.mc-stan.org/t/dev-ops/223

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[bob-carpenter]

Looks good.

[bob-carpenter]

@seantalts --- Can you take a look at what's up with this one? It appears to be dead in travis, though I kicked off the failing tests again.

[seantalts]

Okay, so we think we've seen this issue come to the forefront now that our Windows builds are also using an updated gcc. Bob is working on a fix and we want this to be tested in math (instead of just the upstream Stan windows tests for now), so merging this.