Drop support for 1.8.7.
Drop support for 1.8.7.

Latest push failed in Travis under MRI 1.8.7 due to missing string encoding
facilities. Tried to install 1.8.7 but got few errors; plus, some monkey
patching was on the way to solve the issue. In essence, since 1.8 is EOL, I
decided to just drop support for it (at least for now).

stefanozanella/rupert