Bump reference platform to Ubuntu 18.04 LTS (#3037)

Ubuntu 14.04 reached its end of life on April 30m 2019.
Let's bump the reference to the latest LTS, which is 18.04.

Ubuntu 18.04 has newer Pylint and Cppcheck, the necessary
fixes and suppresses are also included in this PR.

JerryScript-DCO-1.0-Signed-off-by: Csaba Osztrogonác oszi@inf.u-szeged.hu

.travis.yml

@@ -1,8 +1,8 @@
 language: c
 
-# Default environment: Ubuntu Trusty 14.04.
+# Default environment: Ubuntu Bionic 18.04.
 os: linux
-dist: trusty
+dist: bionic
 
 # Default job task: run tests as defined in the $OPT environment variable.
 # Jobs can redefine the 'script' stage in the matrix below.
@@ -12,13 +12,12 @@ script: tools/run-tests.py $OPTS
 matrix:
   include:
     - name: "Checks"
-      install: pip install --user pylint==1.6.5
       script:
         - tools/run-tests.py --check-signed-off=travis --check-doxygen --check-vera --check-license --check-magic-strings --check-pylint
         - travis_wait 40 tools/run-tests.py --check-cppcheck
       addons:
         apt:
-          packages: [doxygen, cppcheck, vera++]
+          packages: [doxygen, cppcheck, vera++, pylint]
 
     - name: "Linux/x86-64 Build, Correctness & Debugger Tests"
       env:
@@ -70,22 +69,20 @@ matrix:
         - OPTS="--quiet --jerry-tests --jerry-test-suite --skip-list=parser-oom.js,parser-oom2.js,stack-limit.js,regression-test-issue-2190.js,regression-test-issue-2258-2963.js,regression-test-issue-2448.js,regression-test-issue-2905.js --buildoptions=--stack-limit=0,--compile-flag=-fsanitize=address,--compile-flag=-m32,--compile-flag=-fno-omit-frame-pointer,--compile-flag=-fno-common,--compile-flag=-O2,--debug,--system-allocator=on,--linker-flag=-fuse-ld=gold"
         - ASAN_OPTIONS=detect_stack_use_after_return=1:check_initialization_order=true:strict_init_order=true
         - TIMEOUT=600
-      compiler: gcc-5
       addons:
         apt:
           sources: ubuntu-toolchain-r-test
-          packages: [gcc-5, gcc-5-multilib]
+          packages: [gcc-multilib]
 
     - name: "UBSAN Tests"
       env:
         - OPTS="--quiet --jerry-tests --jerry-test-suite --skip-list=parser-oom.js,parser-oom2.js --buildoptions=--compile-flag=-fsanitize=undefined,--compile-flag=-m32,--compile-flag=-fno-omit-frame-pointer,--compile-flag=-fno-common,--debug,--system-allocator=on,--linker-flag=-fuse-ld=gold"
         - UBSAN_OPTIONS=print_stacktrace=1
         - TIMEOUT=600
-      compiler: gcc-5
       addons:
         apt:
           sources: ubuntu-toolchain-r-test
-          packages: [gcc-5, gcc-5-multilib]
+          packages: [gcc-multilib]
 
     - name: "Coverity Scan & SonarQube"
       env: