Install trigger for DAO-rescue soft-fork.

Soft-fork should only be primed to trigger if the gas-limit of
block #1760000 is at most 4,000,000.

To accomplish this we pass in the gas limit of that block to
EnvInfo so it can inform Schedule. This gets marshalled through
`OpenBlock`/`ClosedBlock` and the `enact` functions much like
`last_hashes`. `block.rs`'s `env_info()` takes care to ensure
that if the current block happens to be #1760000, then we
populate with the current `gas_limit`.

ethcore/src/tests/helpers.rs

@@ -179,6 +179,7 @@ pub fn generate_dummy_client_with_spec_and_data<F>(get_test_spec: F, block_numbe
 			db,
 			&last_header,
 			last_hashes.clone(),
+			None,
 			author.clone(),
 			3141562.into(),
 			vec![]