test/travis-ci: because of scan-coverity we can't use custom folder - .cmake

since cmake already runned for scan-converity, and we have next message
in log:
> Build files have been written to: /home/travis/build/azat/boostcache
(Oops, there is no ".cmake" at the end)

This patch must fix travic-ci builds after scan-coverity
We also can add condition whether scan-converity is runned, but right
now we don't need this, since this is not a big deal to avoid creating
of ".cmake" folder for builds.

CI: https://travis-ci.org/azat/boostcache/builds/22045206