travis: enable ccache for local builds

For now, it's done for local builds only (i.e. no per-arch Docker
builds yet). The reason is, it's easier to play with ccache when
the compiles are (relatively) fast.

Performance: there is 2x to 3x improvement in build speeds for
ideal cases (same code, hot cache), but the absolute savings
are negligible (example: 7 seconds instead of 23).

Note that ccache is not compatible with gcov, so we only enable it
for non-gcov build (which happens to be the one with clang).

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>