Minor pre Byzantium release 2.2.2 to prevent 2.2.x breaking on ethereumjs-block update

[holgerd77]

This PR narrows down the ethereumjs-block dependency in package.json to ~1.6.0 from ^1.2.2 to keep the dependency version in the 1.6.x range once a 1.7.0 Byzantium release comes out (with changes to block difficulty). The PR has to be accompanied with a minor ethereumjs-vm 2.2.2 release (with a date added to the changelog file).

Rationale:
Once a Byzantium-compatible ethereumjs-block library is released, the 2.2.x (pre-Byzantium) series of ethereumjs-vm install will immediately break (by auto-installing the Byzantium-compatible ethereumjs-block library). This is unfavorable especially in the testing/transition period of Byzantium, when people still might want to install the older version of the library.