Diesel v1.4.4 Support

[MarshallBelles]

Will test and see if there are any breaking changes to move to diesel version 1.4.4

[MarshallBelles]

After the above changes, reading from the DB is working. verified tests complete without issue when removing diesel insert code:

`
test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

 Running target/debug/deps/hstore-8fae5df9d36a3e90

running 1 test
test metadata ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

Doc-tests diesel_pg_hstore

running 7 tests
test src/lib.rs - (line 15) ... ignored
test src/lib.rs - Hstore (line 113) ... ok
test src/lib.rs - (line 87) ... ok
test src/lib.rs - Hstore (line 131) ... ok
test src/lib.rs - Hstore::from_hashmap (line 153) ... ok
test src/lib.rs - (line 34) ... ok
test src/lib.rs - (line 52) ... ok

test result: ok. 6 passed; 0 failed; 1 ignored; 0 measured; 0 filtered out
`

Need to find the cause to:
thread 'metadata' panicked at 'To insert data: DatabaseError(__Unknown, "null value in column \"store\" violates not-null constraint")', tests/hstore.rs:67:5

[MarshallBelles]

thread 'metadata' panicked at 'To insert data: DatabaseError(__Unknown, "null value in column \"store\" violates not-null constraint")', tests/hstore.rs:67:5

Cause of this error was that QueryFragment was not fully implemented in a usable way.
This is now resolved.

[MarshallBelles]

Ready for review.

[MarshallBelles]

@lholden Please review and approve at your convenience.

[SchrodingerZhu]

This can be useful.