Release v4.5.5

• updates to the latest security patch of @slack/client@v3 - thanks @aoberoi (#529)
• adds testing across hubot major versions 2.x and 3.x - thanks @mistydemeo (#525)
• applies a fix to keep users out of the Hubot brain when setting the DISABLE_USER_SYNC environment variable - thanks @jplindquist (#524)