[TASK] Speedup travis build

Different build environments have been set up
mainly to be able to execute lint, unit tests
and functional tests in parallel. However
setting up the build environment on travis
also takes quite some time, so that overall
build times are quite high.

This change cleans up the travis configuration,
reduces the number of builds to only the configured
PHP versions and executes lint, unit tests and
functional test in parallel using the command line
tool "parallel"

By doing so, the overall build time can be reduced
by at least 50%

This change also removes the installation of a
current git version which has been added to
fix temporary Travis service errors.

Releases: 6.2
Change-Id: Ic1591ed0b596df22a648a825639c1a2bbc7ffcc2