Simplify the travis configuration, reduce duplicated rebuilds

The -B make option rebuilds everything from scratch - thus a "make -B",
"make -B test" sequence would first build everything from scratch, then
rebuild (almost) everything and run the tests.

The previous sequence, with "make -B test" without specifying
BUILDTYPE=Debug, meant that it rebuilt a new release build instead
and tested that, thus never actually testing the debug build.

This should reduce the time used for compilation in travis runs
to less than half.

Also explicitly specify BUILDTYPE=Release, for clarity.
Merge pull request #1074 from mstorsjo/simplify-travis

Simplify the travis configuration, reduce duplicated rebuilds