[travis] Add Travis CI service for x86. (#19)

* [CI] Add build status from travis-ci.

* [travis] Use gcc-6 compiler.

* [travis] Use gcc-7 o enable AVX512 compilation.

* [travis] Revert to gcc-6.

* [travis] Disable 128-bit float for the build.

* [travis] Adapt build system to travis environment.

The tester.c sources have been modified to pass the shell environemt
variable to the executable that is invoked with `execvpe`.

A new target "test" have been added to the root Makefile to run the
"iut" tests.

* [travis] Use -lrt for linking clock_gettime.

* [travis] Add -ltr for old gcc.

* [travis] Add a missing -ltr for iutavx build.

* [travis] Fix compilation failure.

It is -lrt, not -ltr...

* [travis] Use travis default behaviour.

* [travis] Revert ENABLEFLOAT128 support for gcc >= 5.