Timezone parsing error in develop branch
I'm running on Ubuntu 16.04 LTS.  I have python I have pulled and checked out develop branch at commit

e079c0db2e2fc28684c4f6a87298d0cf592e6e29

I installed the dependencies in requirements.txt (see attached file)

[pip-list.txt](https://github.com/boto/botocore/files/578672/pip-list.txt)

When I run 

`nosetests tests/unit/`

Then the output ends 

```
======================================================================
FAIL: test_parse_rfc822 (tests.unit.test_utils.TestParseTimestamps)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/mikedd/dev/botocore/tests/unit/test_utils.py", line 249, in test_parse_rfc822
    datetime.datetime(2002, 10, 2, 13, 0, tzinfo=tzutc()))
AssertionError: datetime.datetime(2002, 10, 2, 13, 0, tzinfo=tzlocal()) != datetime.datetime(2002, 10, 2, 13, 0, tzinfo=tzutc())

----------------------------------------------------------------------
Ran 1615 tests in 21.648s

FAILED (failures=1)

```

actual output log attached.  


[nose-unit-run-botocore-develop-e079c0db-log.txt](https://github.com/boto/botocore/files/578670/nose-unit-run-botocore-develop-e079c0db-log.txt)


diff --git a/botocore/utils.py b/botocore/utils.py
index 0f09568ea..bb32a9b8b 100644
--- a/botocore/utils.py
+++ b/botocore/utils.py
@@ -331,7 +331,10 @@ def parse_timestamp(value):
         except (TypeError, ValueError):
             pass
     try:
-        return dateutil.parser.parse(value)
+        # In certain cases, a timestamp marked with GMT can be parsed into a
+        # different time zone, so here we provide a context which will
+        # enforce that GMT == UTC.
+        return dateutil.parser.parse(value, tzinfos={'GMT': tzutc()})
     except (TypeError, ValueError) as e:
         raise ValueError('Invalid timestamp "%s": %s' % (value, e))
 
diff --git a/tests/unit/test_utils.py b/tests/unit/test_utils.py
index 2ef8588a0..188bc0ed0 100644
--- a/tests/unit/test_utils.py
+++ b/tests/unit/test_utils.py
@@ -248,6 +248,18 @@ class TestParseTimestamps(unittest.TestCase):
             parse_timestamp('Wed, 02 Oct 2002 13:00:00 GMT'),
             datetime.datetime(2002, 10, 2, 13, 0, tzinfo=tzutc()))
 
+    def test_parse_gmt_in_uk_time(self):
+        # In the UK the time switches from GMT to BST and back as part of
+        # their daylight savings time. time.tzname will therefore report
+        # both time zones. dateutil sees that the time zone is a local time
+        # zone and so parses it as local time, but it ends up being BST
+        # instead of GMT. To remedy this issue we can provide a time zone
+        # context which will enforce GMT == UTC.
+        with mock.patch('time.tzname', ('GMT', 'BST')):
+            self.assertEqual(
+                parse_timestamp('Wed, 02 Oct 2002 13:00:00 GMT'),
+                datetime.datetime(2002, 10, 2, 13, 0, tzinfo=tzutc()))
+
     def test_parse_invalid_timestamp(self):
         with self.assertRaises(ValueError):
             parse_timestamp('invalid date')
['tests/unit/test_utils.py', 'botocore/utils.py']