Allow pg_config to be overridden.
Improve compatability and ordering in the Makefile.

* Read the distribution name and version from META.json.
* New targets must be defined after `PGXS` is included.
* Add a `dist` target and ignore the file it generates.
Note Makefile changes.
Add CI testing with Travis CI.
Ignore more generated files.
Remove MODULES.

No C files in this extension, and `make` was choking trying to install them.