Release 1.6.0

update for 1.5.1 (crytic#470)

1.5.0.0 -> 1.5.0 (crytic#431)

update hevm to 0.37, echidna 1.4.0.1 (crytic#381)

Update README.md (crytic#375)

echidna 1.3.0.0

Update changelog for release (crytic#331)

Merge pull request crytic#283 from crytic/change_default_to_include_d…

…elay

Default time/block number delays to max of one week

initial stable release (crytic#237)