build against 2.0.0-rc1
Revert "build against 2.0.0-rc1"

It turns out we have to wait for Travis to roll out a new image.
The switch to Ruby version happens before the before_script runs. :(

This reverts commit 55f3199e5c3bcef78cd2aa11dbe480a847303744.
Fixed broken link to SemVer webpage

[ci skip]