Improve travis build matrix

We want to:
 1. Build and run tests with gcc and clang
 2. Check that any combination of OCE_USE_PCH and OCE_COPY_HEADERS_BUILD builds fine

Always build with -j4.  Travis VMs are configured as 32 core machines,
but according to
  https://travis-ci.com/plans#faq
each build has at most 2 cores with 3GB of memory.  Sometimes builds failed
with -j32, they probably ran out of memory.
Build with -j8 instead of -4

See discussion in #514.
Merge pull request #514 from tpaviot/review/travis-build-matrix

Improve travis build matrix