Added check for "shouldSmokeTest" flag.

Checking for this flag allows users to circumvent an issue where smoke
tests run repeatedly, hurting performance/stability. This fixes [Issue #148](https://github.com/sharelatex/sharelatex/issues/148) in part. It requires pull request [Issue #150](https://github.com/sharelatex/sharelatex/pull/150) to establish the necessary settings variable.