Adding notes on building w/different versions of bison

Melbourne 1.1.1.1 won't build with later versions of bison, and it can
be a pain to have multiple versions installed.

The steps added were tested on a fresh Debian vm.
Switch to datarootdir & use /usr/local in README

[skip ci]
Merge pull request #3150 from rubinius/1.8.7-README-updates-bison

Adding notes on building w/different versions of bison