Fix ID token verification (#87)

diff --git a/google/auth/jwt.py b/google/auth/jwt.py
index 7abe4c2..0884b3d 100644
--- a/google/auth/jwt.py
+++ b/google/auth/jwt.py
@@ -96,7 +96,7 @@
 
 def _decode_jwt_segment(encoded_section):
     """Decodes a single JWT segment."""
-    section_bytes = base64.urlsafe_b64decode(encoded_section)
+    section_bytes = _helpers.padded_urlsafe_b64decode(encoded_section)
     try:
         return json.loads(section_bytes.decode('utf-8'))
     except ValueError:
@@ -124,7 +124,7 @@
 
     encoded_header, encoded_payload, signature = token.split(b'.')
     signed_section = encoded_header + b'.' + encoded_payload
-    signature = base64.urlsafe_b64decode(signature)
+    signature = _helpers.padded_urlsafe_b64decode(signature)
 
     # Parse segments
     header = _decode_jwt_segment(encoded_header)