Further issues with new major/minor/fix level parsing #256

• asyncpg version: 0.13
• PostgreSQL version: 10.1
• Python version: 2.7.14-4 / 3.6.4-1
• Platform: Linux Debian sid / Linux Ubuntu Bionic
• Do you use pgbouncer?: No
• Did you install asyncpg with pip?: No (package repository)

In Issue #250 was a fix for Debian branding in version 10.1
But I think it has slight issues and there are more issues with 10.1, not related to the branding but the change of major/minor/fixed version handling.

First of all for the change in #251 :
("PostgreSQL 10.1 (Debian 10.1-3)", (10, 1, 0, 'final', 0),),
I ran into a test fail with this:
E AssertionError: (10, 0, 1, 'final', 0) != ServerVersion(major=10, minor=1, micro=0, releaselevel='final', serial=0)
Following the argument in the comment above the code I think it should instead be:
("PostgreSQL 10.1 (Debian 10.1-3)", (10, 0, 1, 'final', 0),),

Further on I ran into a fail with async def test_server_version_01:

  self.assertEqual(version[:3], (ver_maj, ver_min, ver_fix))

E AssertionError: Tuples differ: (10, 1, 0) != (10, 0, 1)

Again - broken due to Postgresql 10's second number means micro.
That means 10.1 now is:
SELECT current_setting('server_version_num'); => 100001
While 9.5.6 for example was 90506
asyncpg/connection.py:161 get_server_version already does it right, as postgres is doing the mapping.
But test_server_version_01 needs to learn the changed behavior.

That would be solved by:

         version_num = await self.con.fetchval("SELECT current_setting($1)",
                                               'server_version_num', column=0)
         ver_maj = int(version_num[:-4])
-        ver_min = int(version_num[-4:-2])
-        ver_fix = int(version_num[-2:])
+        if ver_maj < 10:
+            ver_min = int(version_num[-4:-2])
+            ver_fix = int(version_num[-2:])
+        else:
+            ver_min = int(version_num[-2:])
+            ver_fix = 0
 
         self.assertEqual(version[:3], (ver_maj, ver_min, ver_fix))