TravisLintBear: Check for internet connection

Add a check for internet connection in the `check_prerequisites`
method without which the bear will fail to run.

Fixes coala#1978

bears/yaml/TravisLintBear.py

@@ -1,3 +1,5 @@
+import requests
+
 from coalib.bearlib.abstractions.Linter import linter
 
 from dependency_management.requirements.GemRequirement import GemRequirement
@@ -25,6 +27,26 @@ class TravisLintBear:
     CAN_DETECT = {'Formatting', 'Syntax'}
     SEE_MORE = 'https://docs.travis-ci.com/user/travis-lint'
 
+    # IP Address of www.google.com
+    check_connection_url = 'http://216.58.218.174'
+    DEFAULT_TIMEOUT = 15
+
+    @classmethod
+    def check_prerequisites(cls):
+        code = cls.get_status_code(
+            cls.check_connection_url, cls.DEFAULT_TIMEOUT)
+        return ('You are not connected to the internet.'
+                if code is None else True)
+
+    @staticmethod
+    def get_status_code(url, timeout):
+        try:
+            code = requests.head(url, allow_redirects=False,
+                                 timeout=timeout).status_code
+            return code
+        except requests.exceptions.RequestException:
+            pass
+
     @staticmethod
     def create_arguments(filename, file, config_file):
         return 'lint', filename

tests/yaml/TravisLintBearTest.py

@@ -1,4 +1,6 @@
 import os
+import requests
+import requests_mock
 from queue import Queue
 
 from coalib.results.Result import Result
@@ -71,3 +73,13 @@ def test_empty_file(self):
                                 file=get_testfile_path(file_name),
                                 severity=RESULT_SEVERITY.NORMAL)],
             filename=get_testfile_path(file_name))
+
+    def test_check_prerequisites(self):
+        with requests_mock.Mocker() as m:
+            m.head(TravisLintBear.check_connection_url,
+                   status_code=200)
+            self.assertTrue(TravisLintBear.check_prerequisites())
+            m.head(TravisLintBear.check_connection_url,
+                   exc=requests.exceptions.RequestException)
+            self.assertTrue(TravisLintBear.check_prerequisites() ==
+                            'You are not connected to the internet.')