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/mbed/Makefile.travis

@@ -27,12 +27,12 @@ install-apt-get-deps:
 
 # Install yotta
 install-yotta:
-	pyenv global 2.7.13 # force the python version to a newer one
 	pip install --user "pyOpenSSL<17.5" # fix for failures with pyOpenSSL 17.5
 	pip install --user yotta
 
 # Perform all the necessary (JerryScript-independent) installation steps.
-install: install-apt-get-deps install-yotta
+install-noapt: install-yotta
+install: install-apt-get-deps install-noapt
 
 
 ## Targets for building Mbed with JerryScript.