Use '~> 3.1.0' instead of '~> 3.0.0'...

...in install instructions for 3.1 maintenance branch
Changed to '~> 3.0' as requested

[ci skip]
Merge pull request #1194 from eliotsykes/patch-1

Use '~> 3.1.0' instead of '~> 3.0.0'...