tests: Gather code coverage data after a successful CI run

tests: Gather code coverage data after a successful CI run

Upon successful completion of a continuous integration (CI) build
through Travis CI, this commit enables coveralls.io to generate
code coverage results by utilizing the gcov data generated
during both the unit and functional test runs.

Note that the m4/ax_code_coverage.m4 file is provided by the
autoconf team, and it has been used verbatim.  The original
file is available here:
https://git.savannah.gnu.org/gitweb/?p=autoconf-archive.git;a=blob_plain;f=m4/ax_code_coverage.m4

Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>
Signed-off-by: Dhaval Giani <dhaval.giani@oracle.com>