Bump default version; add upgrade script

[jasonmp85]

Adding a new UDF means adding an upgrade script to augment existing installations of pg_shard.

fixes #69

[coveralls]

Coverage remained the same at 93.2% when pulling 78ffe34495a5a989e9f5c676f1f1b9eeb959dc7f on feature-upgrade_script-#69 into 9838e1f183223af41260d9ac02e6081dfaaa0c94 on develop.

[jasonmp85]

As a sanity check, I looked over at cstore, which looks exactly like pg_shard will after this Pull Request.

[jasonmp85]

Testing (not currently automated in any fashion):

• Checked out tag v1.0.2 and ran make uninstall; make install. This places the control and 1.0 SQL file in my PostgreSQL extension area
• Checked out this branch and ran make uninstall; make install. This replaces the control file with one defining 1.1 as the default version and adds the 1.1 upgrade (and install) script. This does not remove the 1.0 install script added by the first step
• Modified the init.sql in our tests to first install 1.0 and then upgrade to the latest known version
• Ran tests and verified they passed

Here is the SQL for the upgrade:

CREATE EXTENSION pg_shard VERSION '1.0';
ALTER EXTENSION pg_shard UPDATE;
# \dx
#                                List of installed extensions
#    Name   | Version |   Schema   |                       Description                       
# ----------+---------+------------+---------------------------------------------------------
#  pg_shard | 1.1     | public     | extension for sharding across remote PostgreSQL servers
#  plpgsql  | 1.0     | pg_catalog | PL/pgSQL procedural language
# (2 rows)

[jasonmp85]

So yeah, the two caveats are:

• Future versions don't remove the "main" install script from previous versions
• Our CI system (Travis) isn't currently testing upgrade paths (these could become numerous as time goes on anyhow, so we'll have to see how much support we need for previous versions before we go down the road of automating this).

But this patch works, works with an upgrade from 1.0, and matches what we're doing in cstore.