Safe v0.5.1

Bug Fixes

• Fixes a regression introduced in 0.5.0 with the Spruce
  integration via ~/.svtoken. Safe was writing the wrong key
  value for the Vault address, because I tried to be more
  effficient and didn't notice that it was coming out "url".
  Oops.
• Fix some silly issues with null-context fmt.Errorf calls and
  missing printf format specs, brought to light by the more
  invasive validator of go1.9.

Improvements

• safe is now built against Go 1.9, which should properly plumb
  in support for macOS system certificate pools. All you crazy
  kids with your in-house CAs trusted by your hip macbooks should
  have a better time of using safe without -k!

• The test suite is now run against 0.9.0, but not against 0.8.0
  (we still test against 0.8.3, the latest in the 0.8.x series)