Move (almost all) Travis CI jobs to container-based infrastructure (#2333)

Container-based VMs promise significantly faster boot times.

More or less related changes:
- Added a JOBNAME to all jobs.
- Added `install-noapt` target to several Makefile.travis files to
  avoid `sudo apt-get install ...` steps. Those installations are
  now handled by the apt addon of Travis. The `install` targets are
  kept anyway to keep the makefiles self-contained.
- Removed a legacy workaround from the Coverity Scan job as it
  isn't necessary anymore to fiddle with the cerificates of
  scan.coverity.com.
- Fixed the Mbed and the Zephyr jobs.

JerryScript-DCO-1.0-Signed-off-by: Akos Kiss akiss@inf.u-szeged.hu

targets/riot-stm32f4/Makefile.travis

@@ -30,7 +30,8 @@ install-clone-riot:
 	git clone git://github.com/RIOT-OS/RIOT.git ../RIOT -b 2017.10
 
 # Perform all the necessary (JerryScript-independent) installation steps.
-install: install-apt-get-deps install-clone-riot
+install-noapt: install-clone-riot
+install: install-apt-get-deps install-noapt
 
 
 ## Targets for building RIOT with JerryScript.