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

jerry-debugger/jerry_client.py

@@ -252,7 +252,7 @@ def src_check_args(args):
         print("Error: Non-negative integer number expected: %s" % (val_errno))
         return -1
 
-# pylint: disable=too-many-branches,too-many-locals,too-many-statements,redefined-variable-type
+# pylint: disable=too-many-branches,too-many-locals,too-many-statements
 def main():
     args = jerry_client_main.arguments_parse()
 

jerry-debugger/jerry_client_main.py

@@ -267,7 +267,7 @@ class DebuggerAction(object):
 
 
 class JerryDebugger(object):
-    # pylint: disable=too-many-instance-attributes,too-many-statements,too-many-public-methods,no-self-use,redefined-variable-type
+    # pylint: disable=too-many-instance-attributes,too-many-statements,too-many-public-methods,no-self-use
     def __init__(self, channel):
         self.prompt = False
         self.function_list = {}
@@ -490,7 +490,7 @@ class JerryDebugger(object):
                 index = int(args)
                 if index < 0:
                     print ("Error: A non negative integer number expected")
-                    return
+                    return ""
 
             except ValueError as val_errno:
                 return "Error: Non negative integer number expected, %s\n" % (val_errno)